Complexation of beryllium(II)ion by phosphinate ligands in aqueous solution. Synthesis and XRPD structure determination of Be[(PhPO2)2CH2](H2O)2

Article abstract of DOI:10.1021/ic0261979

Two bifunctional ligands, phenyl(carboxymethyl)phosphinate (ccp^2-) and P,P′-diphenylmethylenediphosphinate (pcp^2-), have been tested as chelating agents of beryllium(II). Both ligands have the same charge and a similar chelating structure, but whereas the 1:1 adduct of pcp^2-, Be(pcp)(H₂O)₂, could be isolated as a white powder, no pure compound could be isolated from solutions containing beryllium(II) and ccp^2-. Instead, the solutions were examined by means of potentiometry and ⁹Be NMR spectroscopy. Analysis of the potentiometric titration data with the program HYPERQUAD suggested the formation of the complex species BeL, [BeHL]⁺, [BeL₂]^2-, and [BeHL₂]^- (L = ccp). The formation constants for these species were determined at 25 °C and / = 0.5 mol dm^-3 NaClO₄. The ⁹Be NMR spectra are consistent with this model. The formation constants found for the ccp^2- complexes are lower than those reported for related phosphonate ligands. However, the effective stability constant (which gives a better indication of the intrinsic coordinating capacity of the ligand at a particular pH) of the complex [Be(ccp)₂]^2- at pH < 4 is greater than the effective constants of the corresponding phosphonoacetate and methylenediphosphonate complexes. The structure of Be(pcp)(H₂O)₂ was determined by X-ray powder diffraction methods and consists of discrete molecules interconnected by an extended 2D network of hydrogen bonds, resulting in a stacking of double layers with a polar core and a lipophilic surface. Crystal data: C₁₃H₁₆BeO₆P₂, fw 339.21, monoclinic P2₁/c₁ a = 16.174(1) A, b = 8.979(1) A, c = 10.929(1) A, β = 90.398(9)°, V = 1587.2(3) A³, Z = 4.

Full text of DOI:10.1021/ic0261979

Inorg. Chem. 2003, 42, 2350−2356
Complexation of Beryllium(II) Ion by Phosphinate Ligands in Aqueous
Solution. Synthesis and XRPD Structure Determination of
Be[(PhPO ) CH ](H O)2
2 2
2
2
,†
,
Franco Cecconi,† Sixto Dominguez,‡ Norberto Masciocchi,§, Stefano Midollini,* Angelo Sironi,* and
Alberto Vacca
|
Istituto della Chimica dei Composti Organometallici, CNR (ICCOM-CNR), Via J. Nardi 39,
I-50132 Firenze, Italy, Departamento di Qu´ımica Inorga´nica, UniVersidad de La Laguna,
E-38204 La Laguna, Tenerife, Spain, Dipartimento di Scienze Chimiche, Fisiche e Matematiche,
UniVersita` dell’Insubria, Via Valleggio 11, I-22100 Como, Italy, Dipartimento di Chimica
Strutturale e Stereochimica Inorganica, UniVersita` di Milano, Via Venezian 21,
I-20133 Milano, Italy, and Dipartimento di Chimica, UniVersita` di Firenze, Via della Lastruccia 3,
I-50019 Sesto Fiorentino, Italy
Received November 20, 2002
2-
2-
Two bifunctional ligands, phenyl(carboxymethyl)phosphinate (ccp ) and P,P′-diphenylmethylenediphosphinate (pcp ),
have been tested as chelating agents of beryllium(II). Both ligands have the same charge and a similar chelating
structure, but whereas the 1:1 adduct of pcp2-, Be(pcp)(H O)2, could be isolated as a white powder, no pure
2
compound could be isolated from solutions containing beryllium(II) and ccp2-. Instead, the solutions were examined
by means of potentiometry and 9Be NMR spectroscopy. Analysis of the potentiometric titration data with the program
HYPERQUAD suggested the formation of the complex species BeL, [BeHL]+, [BeL2]2-, and [BeHL2]- (L ) ccp).
The formation constants for these species were determined at 25 °C and I ) 0.5 mol dm-3 NaClO . The 9Be NMR
4
spectra are consistent with this model. The formation constants found for the ccp2- complexes are lower than
those reported for related phosphonate ligands. However, the effective stability constant (which gives a better
indication of the intrinsic coordinating capacity of the ligand at a particular pH) of the complex [Be(ccp)2]2- at pH
< 4 is greater than the effective constants of the corresponding phosphonoacetate and methylenediphosphonate
complexes. The structure of Be(pcp)(H O)2 was determined by X-ray powder diffraction methods and consists of
2
discrete molecules interconnected by an extended 2D network of hydrogen bonds, resulting in a stacking of double
layers with a polar core and a lipophilic surface. Crystal data: C H BeO P , fw 339.21, monoclinic P21/c, a )
13 16
6 2
3
16.174(1) Å, b ) 8.979(1) Å, c ) 10.929(1) Å, â ) 90.398(9)°, V ) 1587.2(3) Å , Z ) 4.
Introduction
the biochemical mechanism of beryllium toxicity is not yet
properly understood. It is speculated, without convincing
experimental evidence, that the beryllium(II) ion can affect
the function of important proteins by forming a chelated
complex with a hydroxy group of a hydroxy amino acid and
a nearby carboxylate group of a carboxylic amino acid.2 This
hypothesis has stimulated a number of studies of the
complexes formed in aqueous solutions between beryllium-
(II) and ligands containing functional groups such as OH,
COOH, and CO as models for the biochemical systems.4,5
Relatively few investigations have been reported concerning
the interaction of the beryllium ion with molecules containing
residues deriving from phosphate groups though these are
clearly relevant in the biochemical context.6,7
The toxicity of beryllium and beryllium compounds is
well-known, and the effects on human health and the
environment have been evaluated thoroughly.1,2 However,
although the pathology of beryllosis is well documented,3
* To whom correspondence should be addressed. E-mail: midolla@fi.cnr.it
(S.M.); angelo.sironi@istm.cnr.it (A.S.).
† ICCOM-CNR.
‡ Universidad de La Laguna.
§ Universita` dell’Insubria.
Universita` di Milano.
| Universita` di Firenze.
(1) Fields, S. EnViron. Health Perspect. 2001, 109, A474-481.
(2) Skilleter, D. N. Chem. Br. 1990, 26, 26-30.
(3) Saltini, C.; Amicosante, M. Am. J. Med. Sci. 2001, 321, 89-98.
2350 Inorganic Chemistry, Vol. 42, No. 7, 2003
10.1021/ic0261979 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/13/2003

Complexation of Beryllium(II) Ion
Chart 1
Scheme 1
Experimental Section
In the course of our recent investigations of the coordina-
tion chemistry of beryllium in aqueous solution, we found
that bidentate ligands containing carboxylate and/or phos-
phonate groups form stable complexes with the beryllium-
(II) ion. Six-membered chelate rings, such as those formed
by malonate, mal2-, phosphonoacetate, pa3-, and methyl-
enediphosphonate, mdp4- (Chart 1), are more favored than
five- or seven-membered rings; the phosphonate group is
better than carboxylate in regard to coordinating capacity.7-9
This paper concerns the interactions of the beryllium(II)
ion with the two bidentate ligands phenyl(carboxymethyl)-
phosphinate (ccp2-) and P,P′-diphenylmethylenediphosphi-
nate (pcp2-), shown in Chart 1. These ligands both contain
a phosphinate group, -CH2(Ph)PO2; the second donor site
is either a carboxylate group (ccp2-) or a second phosphinate
group (pcp2-).
Safety Note. CAUTION! In View of the extreme toxicity of
beryllium compounds, all experimental work was carried out in a
well Ventilated fume cupboard used exclusiVely for this work. Any
spillage of the beryllium solutions was washed out immediately.
Established procedures for handling dangerous materials were
followed rigorously in all phases of preparation and measurements.
Materials and Methods. Solvents were freshly distilled under
nitrogen from the appropriate drying agents immediately before
use. The P,P′-diphenylmethylenediphosphinic acid, H2pcp, was
prepared according to a previously reported method.11 H2ccp was
synthesized by the reaction in Scheme 1 rather than by the
previously reported procedure.12
Ethylphenylphosphinate, ethylchloroacetate, and phenylphos-
phinic acid (Aldrich) were used as supplied. The IR spectra were
recorded with a Perkin-Elmer 1600 FTIR spectrometer. Thermo-
gravimetric analyses were performed with the aid of a Perkin-Elmer
TG7 instrument, under dynamic N2 flow.
In the case of ccp2-, a series of beryllium(II) complexes
have been characterized in solution by means of potentio-
metry and NMR spectroscopy, but with pcp2-, an insoluble
compound was obtained which was found by X-ray powder
diffraction (XRPD) studies to contain mononuclear chelate
complexes interacting via an extended network of strong
hydrogen bonds. By contrast, first-row transition metal
ionsform polymeric mixed inorganic-organic complexes
with pcp2-.10
Preparation of EtO(O)CCH2P(Ph)(O)OEt. Small pieces of
metallic sodium (0.8 g, 35 mmol) were added in 1 h to a solution
of ethylphenylphosphinate (5.27 g, 31 mmol) in 100 mL of diethyl
ether, at 273 K, with continuous stirring, under an atmosphere of
dry nitrogen. The resulting pale yellow mixture was stirred at room
temperature for 0.5 h and then refluxed for 2 h. The sodium excess
was filtered off, and ethylchloroacetate (3.8 g, 31 mmol) in diethyl
ether (5 mL) was added dropwise to the solution (ca. 0.5 h) at 273
K. The resulting mixture was refluxed for 1 h, then the precipitated
NaCl was filtered off, and the solvent was evaporated to give a
thick oil. This was distilled at 419-424 K (0.5 mmHg) to afford
4.1 g (yield 52%) of EtO(O)CCH2P(Ph)(O)OEt. 31P{1H}(D2O): δ
(4) (a) Alderighi, L.; Gans, P.; Midollini, S.; Vacca, A. Aqueous Solution
Chemistry of Beryllium. In AdVances in Inorganic Chemistry; Sykes,
G., Ed.; Academic Press: New York, 2000; Vol. 50, pp 109-172
and the references therein. (b) Schmidbaur, H. Coord. Chem. ReV.
2001, 215, 223-242. (c) Mederos, A.; Dominguez, S.; Chinea, E.;
Brito, F.; Cecconi, F. J. Coord. Chem. 2001, 53, 191-222.
(5) Cecconi, F.; Ghilardi, C. A.; Ienco, A.; Mariani, P.; Mealli, C.;
Midollini, S.; Orlandini, A.; Vacca, A. Inorg. Chem. 2002, 41, 4006-
4017.
1
33.8 (s) ppm. H NMR(D2O): δ 7.9-7.4 (m, 5H, C6H5), 4.25-
2
3.9 (m, 4H, CH2O), 3.08 (d, 2H, JPH ) 16 Hz, CH2P), 1.31 (t,
3H, CH3), 1.11 (t, 3H, CH3) ppm.
Preparation of HO(O)CCH2P(Ph)(O)OH, H2ccp. HCl (37%,
50 mL) was added to 4 g (15.6) of EtO(O)CCH2P(Ph)(O)OEt, and
the resulting solution was refluxed for 1.5 h. The solvent was
evaporated, and the resulting thick oil of (HO(O)C)CH2(P(Ph)(O)-
OH) was dried in vacuo. Yield 3.02 g, (97%). 31P{1H}(D2O): δ
24.74 ppm.
(6) (a) Barbaro, P.; Cecconi, F.; Dakternieks, D.; Dominguez, S.; Duthie,
A.; Ghilardi, C. A.; Midollini, S.; Orlandini, A.; Vacca, A. Inorg.
Chem. 2001, 40, 2725-2729. (b) Cecconi, F.; Ghilardi, C. A.;
Midollini, S.; Orlandini, A. Inorg. Chem. Commun. 2000, 3, 350-
353. (c) Ciavatta, L.; Iuliano, M.; Porto, R.; Innocenti, P.; Vacca, A.
Polyhedron 2000, 19, 1043-1048. (d) Valle, A.; Chinea, E.; Domingu-
ez, S.; Mederos, A.; Midollini, S.; Vacca, A. Polyhedron 1999, 18,
3253-3256. (e) Bock, J. L.; Ash, D. E. J. Inorg. Biochem. 1980, 13,
105-110. (f) Delpuech, J. J.; Peguy, A.; Rubini, P.; Steinmetz, J. NouV.
J. Chim. 1977, 1, 133-139.
Preparation of K2(ccp). H2ccp (1 g, 5.3 mmol) was dissolved
in 10 mL of H2O, and the solution was adjusted at pH 8.0 with 2
N KOH. Addition of ethanol allowed the precipitation of colorless
microcrystals, which were recrystallized from water/ethanol. Yield
1.29 g (88%). Anal. Found: C, 34.55; H, 2.55. Calcd for
C8H7K2O4P: C, 34.77; H, 2.55. 31P{1H} NMR(D2O): δ 27.74 (s)
(7) Alderighi, L.; Vacca, A.; Cecconi, F.; Midollini, S.; Chinea, E.;
Dominguez, S.; Valle, A.; Dakternieks, D.; Duthie, A. Inorg. Chim.
Acta 1999, 285, 39-48.
1
ppm. H NMR(D2O): δ 7.68-7.33 (m, 5H, C6H5), 2.69 (d,2H,
(8) (a) Alderighi, L.; Cecconi, F.; Ghilardi, C. A.; Mederos, A.; Midollini,
S.; Orlandini, A.; Vacca, A. Polyhedron 1999, 18, 3305-3312. (b)
Alderighi, L.; Bianchi, A.; Mederos, A.; Midollini, S.; Rodriguez, A.;
Vacca, A. Eur. J. Inorg. Chem. 1998, 1209-1215.
2JPH ) 18 Hz, CH2) ppm.
(10) Berti, E.; Cecconi, F.; Ghilardi, C. A.; Midollini, S.; Orlandini, A.;
Pitzalis, E. Inorg. Chem. Commun. 2002, 5, 1041-1043.
(11) Garst, M. E. Synth. Commun. 1979, 9, 261-266.
(9) Barbaro, P.; Cecconi, F.; Ghilardi, C. A.; Midollini, S.; Orlandini,
A.; Alderighi, L.; Peters, D.; Vacca, A.; Chinea, E.; Mederos, A. Inorg.
Chim. Acta 1997, 262, 187-194.
(12) Henning, H. G. J. Prakt. Chem. 1966, 31, 304-311.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2351

Cecconi et al.
Figure 1. (a) Comparison of raw XRPD data (5° < 2θ < 45°) for a conventional sample preparation (as thin layer on a zero-background plate, top) and
a thick, but carefully side-loaded, specimen (bottom). (b) Rietveld refinement plot for Be(pcp)(H2O)2, with difference plots and peak markers at the bottom.
The section above 25° has been enlarged five times.
Chart 2
Preparation of Be(pcp)(H2O)2. A solution of BeSO4‚4H2O
(0.088 g, 0.5 mmol) in H2O (10 mL) was rapidly added to 60 mL
of an aqueous solution of H2pcp (0.148 g, 0.5 mmol) at 353 K.
The mixture was cooled slowly to room temperature and afforded
colorless microcrystals which were filtered, washed with H2O, and
dried in air. Yield 0.292 g, 86%. Anal. Found: C, 45.85; H, 4.80.
Calcd for C13H16BeO6P2: C, 46.03; H, 4.75%. IR (KBr): 2997
(vs, b), 1627 (w, b), 1437 (m), 1357 (w), 1182 (vs), 1141 (vs),
1083 (s), 1072 (s), 1022 (m), 999 (w), 922 (w), 801 (vs), 741 (s),
693 (vs), 590 (m), 522 (s), 488 (w), 466 (w).
orthorhombic) cell of approximate dimensions a ) 16.22 Å, b )
8.97 Å, c ) 10.92 Å, â ) 90.39° [M(20)14 ) 20; F(20)15 ) 45
(0.009, 47)]. Careful visual inspection of the diffraction pattern
indicated that splitting (or broadening) of a few h0l reflections (like
302 and 302h) was consistent with a monoclinic, rather than
orthorhombic, system. Systematic absences indicated P21/c as the
probable space group, later confirmed by successful solution and
refinement.
X-ray Powder Diffraction Analysis. The solid Be(pcp)(H2O)2
containing very thin platelets typically resulted in highly textured
samples, rather unsuitable for structural analysis by X-ray powder
diffraction methods. Therefore, we resorted to a side-loaded
Structure determination was initiated on the data collected on
mounting, which gave a much better diffraction pattern, as shown
the side-loaded specimen by the simulated annealing technique
implemented in TOPAS-R,16 using a flexible PhPO2CH2O2PPh
in Figure 1a. Diffraction data (Cu KR, λ ) 1.5418 Å) were collected
on a vertical scan PW1820 diffractometer, equipped with parallel
(Soller) slits, a secondary beam curved graphite monochromator, a
fragment (with freed rotation about the four independent P-C
bonds, Chart 2). Difference Fourier syntheses (using GSAS17) and
Na(Tl)I scintillation detector, and pulse height amplifier discrimina-
geometrical modeling later afforded approximate coordinates for
tion. The generator was operated at 40 kV and 40 mA. Slits:
the remaining three non-hydrogen atoms. The final refinements were
divergence 1.0°, antiscatter 1.0°, receiving 0.2 mm. Nominal
resolution for the present setup is 0.14° 2θ (fwhm) for the Si(111)
peak at 28.44° (2θ). A long overnight scan on a carefully side-
loaded sample was performed with 5° < 2θ < 105°, with t ) 10
(14) De Wolff, P. M. J. Appl. Crystallogr. 1968, 1, 108-113.
s and ∆2θ ) 0.02°, and used for structure solution and refinement.
Indexing, using TREOR,13 of the low angle diffraction peaks of
a sample prepared as thin layer on a zero-background quartz
monocrystal plate, suggested a primitive monoclinic (but nearly
(13) Werner, P. E.; Eriksson, L.; Westdahl, M. J. Appl. Crystallogr. 1985,
18, 367-370.
(15) Smith, G. S.; Snyder, R. L. J. Appl. Crystallogr. 1979, 12, 60-65.
(16) Kern, A.; Coelho, A. TOPAS-R; Bruker AXS GmbH: Karlsruhe,
Germany, 2001.
(17) Larson, A. C.; Von Dreele, R. B. LANSCE, MS-H805; Los Alamos
National Laboratory: Los Alamos, NM, 1990.
2352 Inorganic Chemistry, Vol. 42, No. 7, 2003

Complexation of Beryllium(II) Ion
Table 1. Decimal Logarithm (log K) Values of the Stepwise Basicity
performed by the Rietveld method with the aid of TOPAS-R, using
a flexible PhPO2CH2O2PPh fragment hinged about a Be(H2O)2
fragment, whose coordination was imposed as tetrahedral by adding
geometric constraints to Be-O contacts. The peak shapes were best
described by the fundamental parameter approach and an isotropic
crystal size broadening factor of Lorentzian contribution. The
background function was modeled by a polynomial function, while
further systematic errors were corrected with the aid of a sample-
displacement angular shift and a preferred orientation model (100
pole, in the March-Dollase18 formulation); a single isotropic
displacement parameter was also refined. The contribution of the
water (and methylene) hydrogen atoms to the scattered intensity
was neglected. The low angle section of the XRPD pattern (2θ <
10°, with one reflection only) was excluded from the final
refinement, being affected by beam overflow and a structured back-
ground, due to contaminating glassy material. Scattering factors,
corrected for real and imaginary anomalous dispersion terms,
were taken from the internal library of TOPAS. Final fractional
coordinates and a full list of bond distances and angles are supplied
as Supporting Information. Crystal data: C13H16BeO6P2, fw
339.21, monoclinic P21/c, a ) 16.174(1) Å, b ) 8.979(1) Å, c )
Constantsa
L
reactionb
malc
pac
mdpc
ccp
pcp
L + H f HL
5.054(2) 8.08(1) 10.41(1) 4.95(1) 3.34(9)
HL+ H f H2L
H2L + H f H3L
H3L + H f H4L
2.51(2)
4.80(1)
0.9(1)
6.90(1) 1.38(3) 1.3(2)
2.75(1)
0.7(2)
a
T ) 298 K and I ) 0.5 mol dm-3 (Me4NCl). Values in parentheses
are standard deviations on the last significant figure. b Charges are omitted
for sake of simplicity. c Data from Alderighi et al.7
which are included in the table. The logarithm of the first
basicity constant for ccp2- is close to the log K1 for mal2-
and to log K2 for pa3-, which were assigned to the
protonation of the carboxylate group.7 Therefore, it is
reasonable to assume that the carboxylate group is protonated
first in ccp2- and the second protonation step is to be ascribed
to the phosphinate group. The ligand pcp2- is significantly
less basic than ccp2- in the first step of protonation because
in pcp2- the protonation takes place at the phenylphosphinate
group. Previous determinations of log K for phenylphosphi-
nates are in the range 2.75-1.85,21 in satisfactory agreement
with the value of log K2 for ccp2-, and both log K1 and log
K2 for pcp2-. On the other hand, the two phosphinate ligands
are much less basic than mdp4-, where protonation occurs
on phosphonate groups, in line with the basicity order
proposed previously,22 phosphonate (first stage) > carboxy-
late > phenylphosphinate. Species distribution diagrams for
ccp2- and pcp2- as a function of pH were obtained using
the HYSS program23 and are given in the Supporting
Information.
A model for the equilibria present in the system beryllium-
(II)-H2ccp under the present experimental conditions was
determined with the aid of the HYPERQUAD program.19
The model includes both the ligand protonation constants
(Table 1) and the equilibrium constants for the hydrolysis
of the beryllium ion.24 The formation constant of each com-
plex species present is shown in Table 2, together with values
for related complexes involving malonate (mal),9 phospho-
noacetate (pa),7 and methylenediphosphonate (mdp).7 The
model for ccp2- is similar to that proposed for the beryllium-
(II) phosphonoacetate system7 with the main difference being
the formation of [BeHL2]-, rather than [Be(HL)2]2-. How-
ever, the trimeric hydroxo species [Be3(OH)3L3]6- and Be3-
(OH)3L were found for L ) pa and were not found for L )
ccp, presumably because of differences in the ligands’
basicity.
10.929(1) Å, â ) 90.398(9)°, V ) 1587.2(3) Å3, Z ) 4, Fcalcd
)
1.419 g cm-3. Final agreement factors Rwp, Rp, and RBragg ) 0.109,
0.077, 0.083, respectively, for 4500 data points collected in the
10° < 2θ < 100° range. The final Rietveld refinement plot is shown
in Figure 1b.
Potentiometric Measurements. Potentiometric determinations
of the equilibrium constants were carried out at 298 K at an ionic
strength of 0.5 mol dm-3 in [Me4N]Cl and with the use of the
HYPERQUAD program.19 The potentiometric apparatus and the
experimental technique have been described previously.7 The
potentiometric titrations were performed by adding a 0.1 mol dm-3
[Me4N]OH solution to acid solutions containing either ligand (in
the deprotonation experiments) or beryllium sulfate and the ligand
(in the complex formation experiments). Concentrations of metal
and ligand were in the ranges 3-6 and 3-4 mmol dm-3
,
respectively. The ligand-to-metal ratios were 1:1, 2:1, and 1:1.5
(four titrations, 134 data points in the pH range 2.1-6.4). The
electrode system was calibrated in terms of hydrogen ion concentra-
tion [H3O+], using the Gran procedure,20 and the log value of Kw
()[H3O+][OH-]) determined for this system is -13.70.
NMR Measurements. The 1H, 31P{1H}, and 9Be NMR spectra
were recorded by a Bruker AC-200 spectrometer operating at
200.130, 81.018, and 28.18 MHz and were referred to internal TMS
and external 85% H3PO4 and aqueous 0.1 mol dm-3 BeSO4,
respectively. Downfield values of the chemical shifts are reported
as positive. If not otherwise specified, the NMR measurements were
carried out in H2O, using coaxial 5 mm tubes, the inner one
containing D2O to provide a lock signal. pH values were measured
using an Orion Research, model 601I apparatus, equipped with a
combined glass-Ag/AgCl microelectrode (Ingold).
The formation constants of the beryllium (II) complexes
with ccp2- are lower than those of the corresponding
Results and Discussion
Solution Studies. The stepwise basicity constants of ccp2-
and pcp2- are shown in Table 1. The values obtained for
ccp2- can be compared with the corresponding values
obtained in previous investigations7,9 for malonate, mal2-,
phosphonoacetate, pa3-, and methylenediphosphonate, mdp4-,
(21) (a) Lubal, P.; Ky´vala, M.; Hermann, P.; Holubova´, J.; Rohovec, J.;
Havel, J.; Lukesˇ, I. Polyhedron 2001, 20, 47-55. (b) Rohovec, J.;
Ky´vala, M.; Vojt´ısˇek, P.; Hermann, P.; Lukesˇ, I. Eur. J. Inorg. Chem.
2000, 195-203. Elesin, A. A.; Zaitsev, A. A.; Karaseva, V. A.;
Nazarova, I. I.; Petukhova, I. V. Radiokhimiya 1972, 14, 374-377.
(22) Lazar, I.; Sherry, A. D.; Ramasamy, R.; Brucher, E.; Kyrali, R. Inorg.
Chem. 1991, 30, 5016-5019.
(23) Alderighi, L.; Gans, P.; Ienco, A.; Peters, D.; Sabatini, A.; Vacca, A.
Coord. Chem. ReV. 1999, 184, 311-318.
(24) Chinea, E.; Dominguez, S.; Mederos, A.; Brito, F.; Sa´nchez, A.; Ienco,
A.; Vacca, A. Main Group Met. Chem. 1997, 20, 11-17.
(18) (a) March, A. Z. Z. Kristallogr. 1932, 81, 285-297. (b) Dollase, W.
A. J. Appl. Crystallogr. 1986, 19, 267-272.
(19) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739-1753.
(20) Gran, G. Analyst 1952, 77, 661-671.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2353

Cecconi et al.
Table 2. Decimal Logarithm (log K) Values of the Beryllium(II)
Complex Formation Constantsa
L
reactionb
malc
pad
mdpd
ccpe
Be + L f BeL
5.36(2)
3.85(3)
9.24(1) 13.7(1) 4.70(1)
5.74(2) 7.66(7) 2.52(3)
4.53(1) 8.3(1) 2.09(5)
3.6(1) 1.5 (1)
BeL + L f BeL2
Be + HL f BeHL
BeL + HL f BeHL2
BeHL + HL f Be(HL)2
Be + H2L f BeH2L
Be + 2 H2L f Be(H2L)2
Be3(OH)3 + L f Be3(OH)3L
≈2
5.0(1)
4.0(2)
7.6(2)
5.26(7)
7.2(3)
Be3(OH)3 + 3L f Be3(OH)3L3 12.84(6) 20.86(3)
a T ) 298 K and I ) 0.5 mol dm-3 (Me4NCl). Values in parentheses
are standard deviations on the last significant figure. b The charges of the
species have been omitted for simplicity. c Data from Barbaro et al.9 d Data
from Alderighi et al.7 e This work.
Figure 3. Distribution diagrams and 9Be NMR spectra at variable pH for
the system BeSO4-H2ccp 1:2, with CBe ) 0.003 mol dm-3 (top) and CBe
) 0.1 mol dm-3 (bottom). The percentages are calculated relative to the
total beryllium concentration. For the sake of clarity, the water molecules
that complete the coordination sphere of the metal ion have been omitted
in the formulas.
between the upper and lower plots resulting from the
difference in total metal and ligand concentrations.
The experimental Be NMR spectra, obtained under the
Figure 2. Plot of the pH dependence of the effective stability constant of
[BeL2]2- (L ) mdp4-, mal2-, pa2-, and ccp2-).
9
complexes with other ligands shown in Table 2. This is also
a reflection of the ligands’ different basicity. In a recent
paper,5 we showed that, because of the competition of metal
and proton for the ligand, it is preferable to compare
coordinating ability in terms of “effective stability constants”,
Keff, as defined by Schwarzenbach.25 The dependence of log
same conditions of concentration, are shown at the right of
Figure 3. The sharp resonance at ca. 0.0 ppm is due to the
[Be(H2O)4]2+ complex. The other broader peaks can be
assigned with the aid of the speciation diagrams, as follows.
The two signals at 0.9 and 1.65 ppm are assigned to the
species BeL(H2O)2 and [BeL2]2-. The downfield shift of the
resonance of the bis-chelate complex is in line with previ-
ously reported spectra.8a The resonance centered at ca. 0.6
ppm (pH ) 2.13) may be due to the two rapidly exchanging
species [BeHL (H2O)3]+ and BeL(H2O)2, eq 1.
K
eff for the [BeL2]2+ complexes is illustrated in Figure 2. It
can be seen that at high pH, where the ligands are completely
deprotonated, the trend in effective constants is the same as
the trend in stability constants. At lower pH values, where
the ligand is partially protonated, the order changes, and it
is particularly noteworthy that at pH < 4 the ligand ccp2-
exhibits a high coordination ability, second only to malonate.
The species distribution diagrams shown in Figure 3 were
constructed using the HySS program.23 The percentages of
the various species were calculated using the equilibrium
constants shown in Tables 1 and 2, the solubility product of
beryllium hydroxide (Ksp ) [Be2+][H+]-2 ) 106.87) reported
by Bruno,26 and a metal-to-ligand ratio of 1:2. Beryllium
hydroxide precipitates at pH values above ca. 6. The two
concentrations that were used correspond to some experi-
mental conditions. Note that there is considerable difference
[BeHL(H2O)3]+ / BeL(H2O)2 + H3O+
(1)
The corresponding 31P{1H} NMR spectra have been found
to be consistent. They show two resonances at δ 31.5 and
31.2 ppm attributable to BeL(H2O)2 and [BeL2]2-, respec-
tively, together with a high field signal due to the rapidly
exchanging, uncoordinated, species H2L, HL-, and L2-.
The clearest difference between the speciation at the two
concentrations occurs at high pH values. In both cases, the
two species BeL(H2O)2 and [BeL2]- are present together,
but [BeL2]- predominates when the concentration is high,
whereas BeL(H2O)2 predominates when the concentration
is low. This behavior is expected as dilution strongly favors
dissociation.
(25) Schwarzenbach, G. Die komplexometrische Titration; Ferdinand Enke
Verlag: Stuttgart, 1955; p 9.
(26) Bruno, J. J. Chem. Soc., Dalton Trans. 1987, 2431-2437.
2354 Inorganic Chemistry, Vol. 42, No. 7, 2003

Complexation of Beryllium(II) Ion
Solid State Studies. H2ccp reacts with Be(OH)2 (molar
ratios 1:1 or 2:1) under continuous stirring, to yield colorless
solutions. Evaporation of the solvent, after adjusting the pH
with ammonia or KOH, always produced amorphous solids
whose elemental analyses and NMR spectra were not
consistent with any pure beryllium compound.
H2pcp reacts with BeSO4 in aqueous solution at 353 K to
give a metastable solution27 from which irregularly shaped
microcrystals of formula Be(pcp)(H2O)2 slowly separate. This
compound is insoluble in water and common organic
solvents, so that single crystals suitable for a conventional
structural analysis could not be prepared. It decomposes
without melting upon heating. It was characterized by X-ray
powder diffraction and solid-state IR spectroscopy. The IR
spectrum shows a very strong, broad band centered at 3000
cm-1, due to the OH stretching of H2O. Both the broadness
and the shift of the band toward lower energies are consistent
with the presence of an extended hydrogen bond network.
Crystals of Be(pcp)(H2O)2 contain discrete monomeric
molecules, with the Be(II) ion tetrahedrally coordinated by
two water molecules and one chelating pcp ligand (see Figure
4a). The six-membered ring formed by chelation is slightly
puckered toward a boat conformation, with torsional angles
defined by the two Cipso-P-CH2-P sequences of 70.2° and
87.3°. The two phenyls attached to the phosphorus atoms
possess slightly different conformations about the P-Cipso
vectors (OBe-P-Cipso-C 6.3° vs 10.2°) and lie on the same
side of the molecule, opposite to the Be(H2O)2 fragment;
therefore, each molecule possess two distinct contact sur-
faces, of lipophilic and hydrophilic character, respectively.
Accordingly, in the crystal, aryls of different molecules face
each other, while water molecules and oxygen atoms of the
phosphinic residue extensively interact through a complex,
two-dimensional, hydrogen bond network (O1w‚‚O3 2.44
Å; O1w‚‚‚O2 2.71 Å, O2w‚‚‚O2 2.47 Å; O2w‚‚‚O3 2.86 Å;
note that only the terminal O(P) atoms act as hydrogen bond
acceptors, thus uniquely determining the H-atom locations).
The resulting crystal structure is therefore built up from two-
dimensional double layers, stacked along the crystallographic
a axis, where the polar residues are deeply buried in the
bilayer core (see Figure 4b). The very poor solubility of this
species in water and organic solvents initially suggested a
polymeric structure,28 possibly built around the same struc-
tural motifs, but with O-P(O)Ph-CH2-Ph(O)P-O ligands
linking Be(H2O)2 fragments in the exo-, rather than endo-
bidentate mode. The present structure determination clearly
ruled out the presence of fully stretched phosphinates and
revealed the stereochemistry of a monomeric compound,
similar to the Be[salycilate](H2O)2 species reported by
Schmidbaur et al. in 1991.29 Interestingly, also the latter
species contains several H-bonded interactions, and a double
Figure 4. (a) Drawing of the molecular structure of Be(pcp)(H2O)2. (b)
Crystal packing viewed down [001], highlighting the bilayer structure,
stacking along a (vertical axis); for sake of simplicity, hydrogen atoms have
been omitted.
layer structure very similar to ours (although the lipophilic
part is somewhat smaller), but is slightly more soluble,
1
9
allowing H and Be NMR characterization in solution.
When Be(pcp)(H2O)2 is heated in a nitrogen gas stream,
it loses water in the range 110-150 °C, and the dehydrated
species Be(pcp) is produced. A complete structural charac-
terization of this material was not possible, since its XRPD
pattern showed only one very strong peak at 2θ ) 6.21°,
two diffuse halos centered at ca. 17 and 24°, and two weak
(second and third) harmonics of the major peak. The
corresponding d-value, of ca. 14.2 Å, needs to be compared
to the 16.2 Å value of the parent species and suggests that
loss of water preserves the bilayer structure (even if shrunk
by ca. 2 Å along the a direction), accompanied by amor-
phization in the bc plane. Moreover, we have found that the
powders of the anhydrous material are stable as such if left
in contact with humid atmosphere and do not restore the
original hydrated crystalline phase.
(27) A 9Be NMR spectrum of this solution showed a singlet (w1/2 ) 7 Hz)
at δ 0.344 ppm, indicating the presence of only one beryllium(II)
species.
(28) Polymeric complexes prepared by direct reaction of beryllium acety-
lacetonate and monophosphinic acids had been characterized about
30 years ago. See: Block, B. P. Inorg. Macromol. ReV. 1970, 1, 115-
125.
(29) Schmidbaur, H.; Kumberger, O.; Riede, J. Inorg. Chem. 1991, 30,
3101-3103.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2355

Cecconi et al.
Conclusions
approximations or other geometrical constraints. Accordingly,
several problematic structures of polycrystalline substances
have been recently determined by the joint use of XRPD
methods and ancillary (geometric and/or energetic) informa-
tion, including cases of unknown stoichiometry30 and of
wrong stereochemistry,31connectivity,32 and geometry.33
The bifunctional ligands ccp2- and pcp2- have been found
to be capable of coordination to the beryllium(II) ion. In the
case of ccp2-, both mono-chelated and bis-chelated com-
plexes have been characterized in aqueous solution. Because
of the low basicity of the -PO2Ph donor group, the formation
constants of the complexes are lower than those reported
for the complexes of the related phosphonate ligands pa3-
and mdp4- (Chart 1). However, it is noteworthy that at pH
< 4 the effective stability constant of the complex [Be(ccp)2]2-
is greater than those of the corresponding phosphonoacetate
and methylenediphosphonate derivatives. Accordingly, de-
spite its low basicity, the -PO2Ph group can form the
monodentate complex [Be(Hccp)(H2O)3]+ at low pH values
even without the assistance of a chelate.
Acknowledgment. We gratefully acknowledge the fi-
nancial support of the Italian Ministero dell’Ambiente for
the contract PR1/C. A.V., A.S., and N.M. are grateful to the
Italian MIUR for financial support. A.S. and N.M. thank the
Chamber of Commerce of Como for funding. They also
acknowledge Drs. A. Kern and A. Coelho for providing the
highly performing â version of the TOPAS-R program. S.D.
thanks the Education Council of the Government of the
Canary Islands for financial support.
In the case of the ligand pcp2-, the insolubility of the
complex Be(pcp)(H2O)2 prevents the formation of other
derivatives. The structure determination of Be(pcp)(H2O)2
performed on a highly textured polycrystalline material
shows that conventional laboratory techniques can provide
useful stereochemical information even on moderately
complex species (22 crystallographically independent non-
hydrogen atoms). A distinctive feature of this study is that,
despite the obvious lack of resolution, analysis of the
intermolecular contacts has allowed the unambiguous as-
signment of elusive (water) H-atoms. It is well-known that
such low-resolution XRPD structure analyses cannot provide
detailed molecular information without the use of rigid body
Supporting Information Available: Speciation diagrams for
pcp2- and ccp2- protonation. Final fractional coordinates and full
list of bond distances and angles. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC0261979
(30) Cariati, E.; Ugo, R.; Cariati, F.; Roberto, D.; Masciocchi, N.; Galli,
S.; Sironi, A. AdV. Mater. (Weinheim, Ger.) 2001, 13, 1665-1668.
(31) Masciocchi, N.; Moret, M.; Cairati, P.; Ragaini, F.; Sironi, A. J. Chem.
Soc., Dalton Trans. 1993, 471-475.
(32) Masciocchi, N.; Ragaini, F.; Cenini, S.; Sironi, A. Organometallics
1998, 17, 1052-1057. Masciocchi, N.; Ragaini, F.; Sironi, A.
Organometallics 2002, 21, 3489-3492.
(33) Li, J.; McCulley, F.; McDonnell, S. L.; Masciocchi, N.; Proserpio, D.
M.; Sironi, A. Inorg. Chem. 1993, 32, 4829-4833.
2356 Inorganic Chemistry, Vol. 42, No. 7, 2003