Stabilization of hypophosphite in the binding pocket of a dinuclear macrocyclic complex: Synthesis, structure, and properties of [Ni 2L(μ-O2PH2)]BPh4 (L = N 6S2 Donor Ligand)

Article abstract of DOI:10.1021/ic301710b

The dinickel(II) complex [Ni₂L(ClO₄)]ClO₄ (1), where L^2- represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [nBu₄N]H ₂PO₂ to form the hypophosphito-bridged complex [Ni ₂L(μ-O₂PH₂)]⁺, which can be isolated as an air-stable perchlorate [Ni₂L(μ-O₂PH ₂)]ClO₄ (2) or tetraphenylborate [Ni₂L(μ- O₂PH₂)]BPh₄ (3) salt. 3·MeCN crystallizes in the triclinic space group P1. The bisoctahedral [Ni ₂L(μ-O₂PH₂)]⁺ cation has a N ₃Ni(μ_1,3-O₂PH₂)(μ-S) ₂NiN₃ core structure with the hypophosphito ligand attached to the two Ni^II ions in a μ_1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni₂L]²⁺ dication for more basic oxoanions. Treatment of [Ni₂L(μ-O₂PH ₂)]ClO₄ with H₂O₂ or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for 3 reveals the presence of ferromagnetic exchange interactions between the Ni^II (S = 1) ions with a value for the magnetic exchange coupling constant J of +22 cm^-1 (H = -2JS ₁S₂). These results are additionally supported by DFT (density functional theory) calculations.

Full text of DOI:10.1021/ic301710b

Article
pubs.acs.org/IC
Stabilization of Hypophosphite in the Binding Pocket of a Dinuclear
Macrocyclic Complex: Synthesis, Structure, and Properties of
[Ni2L(μ‑O2PH2)]BPh4 (L = N6S2 Donor Ligand)
Jochen Lach,† Alexander Jeremies,† Vasile Lozan,†,§ Claudia Loose,‡ Torsten Hahn,‡ Jens Kortus,‡
and Berthold Kersting*,†
†Institut fur Anorganische Chemie, Universitat Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
̈
̈
̈
‡Institut fur Theoretische Physik, TU Bergakademie Freiberg, Leipziger Str. 23, 09596 Freiberg
S
* Supporting Information
ABSTRACT: The dinickel(II) complex [Ni2L(ClO4)]ClO4
(1), where L2− represents a 24-membered macrocyclic
hexaamine-dithiophenolate ligand, reacts with [nBu4N]H2PO2
to form the hypophosphito-bridged complex [Ni2L(μ-
O2PH2)]+, which can be isolated as an air-stable perchlorate
[Ni2L(μ-O2PH2)]ClO4 (2) or tetraphenylborate [Ni2L(μ-
O2PH2)]BPh4 (3) salt. 3·MeCN crystallizes in the triclinic
space group P1. The bisoctahedral [Ni L(μ-O PH )]+ cation
̅
2
2
2
has a N3Ni(μ1,3-O2PH2)(μ-S)2NiN3 core structure with the hypophosphito ligand attached to the two NiII ions in a μ1,3-bridging
mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni2L]2+ dication
for more basic oxoanions. Treatment of [Ni2L(μ-O2PH2)]ClO4 with H2O2 or MCPBA results in the oxidation of the bridging
thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection
offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for 3 reveals the presence
of ferromagnetic exchange interactions between the NiII (S = 1) ions with a value for the magnetic exchange coupling constant J
of +22 cm−1 (H = −2JS1S2). These results are additionally supported by DFT (density functional theory) calculations.
N6S2 donor set and L′ is a main group or transition metal
oxoanion. The X-ray crystal structures of the following
complexes have been reported: [Ni2L(μ-PO2(OH)2)]BPh4,12
[Ni2L(μ-SO4)],13 [Ni2L(μ-ClO4)]BPh4,13 [Co2L(μ-MoO4)],14
[Ni2L(μ-CrO4)],13 [Ni2L(μ-MoO4)],13 [Co2L(μ-MoO2(OH)-
(OMe))]+,14 [Ni2L(μ-PO2(OC6H4-pNO2)2)]+,15 and [Ni2L(μ-
VO2(OMe)2)]+.16 The oxo- and oxo-alkoxo anions are all
coordinated in a μ1,3-bridging O2E(OR)2 mode thereby
generating M2(μ-O2E(O)2), M2(μ-O2E(-OR)2), or M2(μ-
O2E(O)(OR)) structures with two oxo, hydroxido (R = H),
or alkoxo (R = alkyl) groups, respectively, which are deeply
buried in the pocket provided by the supporting ligand.
INTRODUCTION
■
−
Several complexes containing the hypophosphite ion (H2PO2 )
as a ligand have been reported and crystallographically
characterized. These include structures of d-block,1−3 s-
block,4 p-block,5 lanthanide,6,7 and actinide metals.8 The
hypophosphito ligand in these metal complexes is very flexible
in its coordination (Figure 1). The doubly bridging μ2,5,8,9
To further develop the coordination chemistry of these
complexes, we decided to prepare and characterize a [Ni2L(μ-
L′)]+ complex bearing a H2PO2 ion as a coligand. The
−
Figure 1. Coordination modes of the hypophosphito ligand.
motivation for this work is as follows. First, nickel(II)
hypophosphito complexes are not very stable, a behavior
which is taken advantage of in electroless nickel plating.17,18
Second, there exists only little information in the literature
triply bridging μ3,1,3 and quadruply bridging μ4 modes2,4 are all
−
represented. The behavior of the H2PO2 ion in these
−
regarding the bonding of a H2PO2 ion in dinuclear nickel(II)
compounds is expected to be interesting due to its redox
behavior. In fact, the hypophosphite is sensitive toward
oxidizing agents, and oxidation to phosphite (HPO32−) or
phosphate (PO43−) has been reported in several cases.7,10,11
Recently, we have been exploring the chemistry of
bisoctahedral transition metal complexes of the type [M2L-
(L′)]n+ where L2−represents a supporting macrocycle with an
−
complexes. Third, incorporation of the H2PO2 ion in the
binding pocket of the [Ni2L]2+ complex might lead to an
alteration of its reactivity. Finally, the type and magnitude of
Received: August 3, 2012
Published: November 5, 2012
© 2012 American Chemical Society
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Article
−
−
[ν3(ClO4 )], 1041 (m) [νsymm(PO2)], 1002 (s), 981 (w), 928 (w),
magnetic coupling via a H2PO2 unit is scarce. Herein, we
report on the successful preparation and isolation of the
complex salts 2 and 3 containing the hypophosphito-bridged
dinickel(II) complex cation [Ni2L(μ-O2PH2)]+ (Figure 2). The
crystal structure, electrochemistry, reactivity features, magnetic
properties, and DFT calculations of 3 are also described.
912 (w), 882 (w), 826 (s), 817 (w), 810 (s), 751 (w), 625 (s)
−
[ν4(ClO4 )], 563 (w), 535 (w), 483 (m), 442 (w), 434 (w), 425 (w),
416 (w). Electrospray ionization mass spectrometry (ESI(+)-MS): m/
z (CH3CN) = 849.3 [M]+. Anal. Calcd for C38H66ClN6Ni2O6PS2
(950.91): C 48.00, H 7.00, N 8.84, S 6.74. Found: C 47.88, H 7.14, N
8.74, S 6.63. Cyclic voltammetry (CH3CN): E/V (ΔEp/ V) vs SCE:
0.50 V (0.09 V), 1.24 V (0.12 V).
[Ni2L(μ-O2PH2)]BPh4 (3). A solution of NaBPh4 (342 mg, 1.00
mmol) in methanol (2 mL) was added to a solution of 2 (95 mg, 0.10
mmol) in methanol (40 mL) and stirred for 1 h. The resulting bright
green precipitate was filtered, washed with methanol, and dried in air
to give 107 mg (91%) of 3 as a green, air-stable, microcrystalline
powder. Mp 319−320 °C (decomp.). UV−vis (CH3CN): λmax/nm (ε/
M−1 cm−1) = 304 (9679), 666 (20), 1133 (53). IR (KBr, cm−1): ν
̃
=
3159 (vw), 3124 (vw), 3053 (s), 3041 (sh), 3030 (sh), 3013 (sh),
2999 (m), 2982 (w), 2962 (s), 2934 (w), 2900 (m), 2873 (s), 2334
(s), 2300 (m), 2272 (w), 1946 (m), 1881 (m), 1820 (m), 1761 (w),
1707 (m), 1625 (m), 1581 (m), 1562 (w), 1478 (vs), 1459 (s), 1426
(s), 1382 (m), 1362 (w), 1349 (vw), 1324 (vw), 1305 (m), 1265 (m),
1232 (w), 1191 (s, br), 1171 (vw), 1152 (s), 1130 (vw), 1107 (vw),
1086 (m), 1075 (s), 1067 (m), 1058 (sh), 1039 (sh), 1033 (s), 1001
(m), 979 (m), 927 (w), 910 (w), 881 (s), 861 (w), 847 (m), 824 (m),
809 (s), 741 (w), 733 (s), 706 (vs), 625 (w), 610 (m), 563 (m), 540
(w), 532 (vw), 481 (w), 467 (w), 416 (w). Electrospray ionization
mass spectrometry (ESI(+)-MS): m/z (CH3CN) = 849.3 [M]+. Single
crystals of 3·MeCN suitable for an X-ray structure analysis were grown
by slow evaporation of an ethanol/acetonitrile solution of 3. These
crystals slowly lose solvent molecules of solvent of crystallization at
ambient temperature and turn dull. Anal. Calcd (%) for
C62H86BN6Ni2O2PS2·3H2O (1170.69 + 54.05): C 60.80, H 7.57, N
6.86, S 5.24. Found: C 60.70, H 7.28, N 6.81, S 5.37.
Figure 2. Chemical structure of the [Ni2L(μ-O2PH2)]+ complex in the
salts 2 and 3.
EXPERIMENTAL SECTION
■
General. Unless otherwise noted the preparations were carried out
under an argon atmosphere by using standard Schlenk techniques. The
complex salt [Ni2L(μ-ClO4)]ClO4 (1) was prepared as described in
the literature.13 Melting points were determined in capillaries and are
uncorrected. The infrared spectra were recorded as KBr discs using a
Bruker Tensor 27 FT-IR-spectrophotometer. Electronic absorption
spectra were recorded on a Jasco V-670 UV−vis−NIR spectropho-
tometer. Elemental analysis were carried out on a VARIO EL
elemental analyzer. ESI mass spectra were recorded on a Bruker
Daltronics ESQUIRE3000 PLUS spectrometer. Cyclic voltammetry
measurements were carried out at 25 °C with an EG&G Princeton
Applied Research potentiostat/galvanostat model 263 A. The cell
contained a Pt working electrode, a Pt wire auxiliary electrode, and a
Ag wire as reference electrode. Concentrations of solutions were 0.10
M in supporting electrolyte ([n-Bu4N]PF6) and ca. 1 × 10−3 M in
sample. Cobaltocene was used as internal standard.19 All potentials
were converted to the SCE reference. Temperature-dependent
magnetic susceptibility measurements on powdered solid samples of
compound 3 were carried out to study the magnetic exchange
interactions in the dinuclear NiII complex. The susceptibility data of 3
were measured between 2 and 330 K using a MPMS 7XL SQUID
magnetometer (Quantum Design) in an applied external magnetic
field of 0.5 T.
The complexes 4 and 5 below have been reported previously, but
were obtained here in another fashion, by substitution of the
hypophosphito coligand in 2 (see Scheme 1). The earlier method
used the chloro-bridged complex [Ni2L(μ-Cl)]ClO4 as starting
material.12,20
Scheme 1. Carboxylate Exchange and S-Oxygenation
Reactions of Compound 2
Caution! Perchlorate salts of transition metal complexes are hazardous
and may explode. Only small quantities should be prepared and great care
should be taken.
[Ni2L(μ-O2PH2)]ClO4 (2). To a solution of 1 (92 mg, 0.10 mmol) in
methanol (30 mL) was added with stirring a solution of (nBu4N)-
H2PO2 (82.5 mg, 0.268 mmol) in methanol (5 mL) at room
temperature. The reaction mixture was stirred for about 4 h after
which time solid LiClO4·3H2O (160 mg, 1.00 mmol) was added. The
solution was concentrated under reduced pressure until bright green
crystals formed, which were filtered, washed with cold methanol, and
dried in air. The crude product was purified by recrystallization from a
mixed acetonitrile/ethanol solution. Yield: 73 mg (77%). UV−vis
(CH3CN): λmax/nm (ε/M−1 cm−1) = 671 (24), 1134 (73). Mp 312−
[Ni2L(μ-O2CCH3)]ClO4 (4). To a solution of 2 (95 mg, 0.10 mmol)
in methanol (30 mL) was added with stirring a solution of sodium
acetate (82 mg, 1.00 mmol) in methanol (10 mL) at room
temperature. The reaction mixture was stirred for about 24 h after
which time solid LiClO4·3H2O (160 mg, 1.00 mmol) was added. The
solution was concentrated under reduced pressure until bright green
crystals formed, which were filtered, washed with cold methanol, and
dried in air. The crude product was purified by recrystallization from a
mixed acetonitrile/ethanol solution. Yield: 79 mg (83%). IR (KBr,
314 °C (decomp). IR (KBr, cm−1): ν
̃
= 3439 (m), 3042 (w), 2993
(w), 2957 (s), 2900 (m), 2868 (s), 2806 (w), 2780 (w), 2332 (m)
[νs(PH2)], 2298 (m) [νas(PH2)], 2273 (w), 2163 (w), 1618 (w), 1603
(vw), 1527 (w), 1482 (w), 1461 (s), 1422 (w), 1393 (w), 1363 (m),
1349 (w), 1323 (w), 1307 (m), 1264 (m), 1232 (m), 1195 (s)
[νas(PO2)], 1169 (w), 1156 (w), 1098 (vs), 1087 (vs), 1076 (vs),
cm−1): ν
̃
= 3442 (m), 2962 (m), 2870 (m), 2802 (w), 1589 (s,
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−
−
νas(RCO2 )), 1459 (s), 1427 (m, νs(RCO2 )), 1396 (w), 1363 (w),
1308 (w), 1266 (w), 1233 (w), 1202 (w), 1170 (w), 1153 (w), 1097
(s), 1080 (s), 1058 (m), 1039 (m), 1002 (w), 983 (w), 931 (w), 912
(w), 890 (w), 880 (w), 826 (m), 817 (w), 807 (w), 752 (w), 662 (w),
624 (m), 564 (w), 534 (w), 492 (w), 417 (w). Electrospray ionization
mass spectrometry (ESI(+)-MS): m/z (CH3CN) = 843.2 [M]+. UV−
vis (MeCN): λmax/nm (ε/M−1 cm−1) = 650 (26), 1131 (67). These
data are identical with those reported in the literature.12
SMART.21 Data processing was accomplished with SAINT.22 An
empirical absorption correction was performed with the program
SADABS.23 The structure was solved by direct methods in SHELXS24
and refined by full-matrix least-squares on F2 using SHELXTL (version
5.1).25 All non-hydrogen atoms were refined with anisotropic
displacement parameters. The hydrogen atoms were placed at
calculated positions, using appropriate riding or rotation models
(HFIX 23 for CH2, HFIX 43 for aromatic CH, and HFIX 133 for
CH3), and assigned isotropic thermal parameters (e.g., Uiso(H) = 1.2 ×
Uiso(parent) or Uiso(H) = 1.5 × Uiso(C) for methyl groups). The
hydrogen atoms of the hypophosphite were placed at ideal positions
assuming a P−H distance of 1.40 Å and a tetrahedral geometry and
were given isotropic thermal parameters 1.2 times the thermal
parameter of the phosphorus atom. Both tert-butyl groups of the
supporting ligand were found to be rotationally disordered. A split
atom model with restrained C−C and C···C distances was applied
using SADI instructions implemented in SHELXTL. The site
occupancies of the two positions were refined as follows: C(32a)−
C(33a) /C(32b)−C(34b): 0.71(1)/0.29(1), and C(36a)−C(38a)
/C(36b)−C(38b): 0.56(1)/0.44(1). Graphics were obtained with
ORTEP 3 for Windows.26
[Ni2L(μ-O2CPh)]ClO4 (5). To a solution of 2 (95 mg, 0.10 mmol) in
methanol (30 mL) was added with stirring a solution of sodium
benzoate (144 mg, 1.00 mmol) in methanol (10 mL) at room
temperature. The reaction mixture was stirred for about 24 h after
which time solid LiClO4·3H2O (160 mg, 1.00 mmol) was added. The
solution was concentrated under reduced pressure until bright green
crystals formed, which were filtered, washed with cold methanol, and
dried in air. The crude product was purified by recrystallization from a
mixed acetonitrile/ethanol solution. Yield: 75 mg (79%). IR (KBr,
cm−1): ν
̃
= 3445 (m), 2965 (m), 2897 (m), 2868 (m), 2806 (w), 1600
(m), 1586 (m), 1569 (s), 1463 (s), 1428 (s), 1406 (m), 1363 (m),
1308 (w), 1263 (w), 1232 (w), 1202 (w), 1172 (w), 1153 (w), 1096
(s), 1057 (m), 1040 (m), 1001 (w), 982 (w), 931 (w), 913 (w), 882
(w), 826 (m), 753 (w), 708 (w), 675 (w), 623 (m), 564 (w), 538 (w),
493 (w), 471 (w), 443 (w), 418 (w). Electrospray ionization mass
spectrometry (ESI(+)-MS): m/z (CH3CN) = 905.3 [M]+. UV−vis
(MeCN): λmax/nm (ε/M−1 cm−1) = 651 (29), 1123 (69). These data
are identical with those reported in the literature.20
Crystallographic data for 3·MeCN: C64H89BN7Ni2O2PS2 (1211.74 g
mol−1), triclinic, space group P1, a = 13.179(3) Å, b = 15.736(3) Å, c =
̅
16.799(3) Å, α = 108.48(3)°, β = 91.90(3)°, γ = 107.61(3)°, V =
3117.1(11) Å3, Z = 2, ρ = 1.291 g cm−3, μ(Mo Kα) = 0.745 mm−1,
crystal size 0.18 × 0.15 × 0.10 mm3, θ limits 1.29−28.35°. Of 28 567
measured reflections (Rint = 0.0399), 14 655 were unique and 7874
observed (I > 2σI). Final residuals: R1 = 0.0401, wR2 = 0.0829 (for
observed data), R1 = 0.0987, wR2 = 0.1234 for all data. Max, min
peaks: 0.418, −0.620 e/Å3. CCDC-870563 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[Ni2L′(μ-O2PH2)]ClO4 (6), Where L′ is the Oxidized Hexaza-
(disulfonato) Form of (L)2− (see Scheme 1). Method A (Oxidation
with Hydrogen Peroxide). To a solution of 2 (95 mg, 0.10 mmol) in
methanol (30 mL) was added with stirring an excess of hydrogen
peroxide (0.28 mL, 50 wt % solution in water, 5.00 mmol) at room
temperature. The reaction mixture was stirred for about 7 d after
which time solid LiClO4·3H2O (160 mg, 1.00 mmol) was added. The
solution was concentrated under reduced pressure until pale-green
crystals formed, which were filtered, washed with cold methanol, and
dried in air. The crude product was purified by recrystallization from a
mixed acetonitrile/ethanol solution. Yield: 75 mg (71%). IR (KBr,
Electronic Structure Calculations. The density functional calcu-
lations of the free, isolated [Ni2L(μ-O2PH2)]+ cation (in 3) have been
carried out using the Naval Research Laboratory Molecular Orbital
Library (NRLMOL) program package. NRLMOL is an all-electron
implementation of DFT including large Gaussian orbital basis sets,
numerically precise variational integration and an analytic solution of
Poisson’s equation in order to accurately determine the self-consistent
potentials, secular matrix, total energies, and Hellmann−Feynman−
Pulay forces.27−34 The calculations have been carried out using the
generalized gradient approximation of Perdew, Burke, and Enzerhof
(PBE) for the exchange and correlation functional.35,36 For
comparison we carried out calculations using the ORCA37,38 program
package (revision 2.80) including the B3LYP39 functional and Ahlrichs
triple-ζ valence basis set.40 Starting from the experimental X-ray
structure we generated a molecular model of the isolated complex
cation which has been used for the calculation. The magnetic exchange
coupling itself was obtained by mapping the energy difference between
the high-spin (S = 2) and a low-spin (S = 0) configuration of the
molecule to the H = −2JS1S2 Heisenberg Hamiltonian. The low-spin
configuration was obtained using the broken symmetry approach
detailed previously.41
cm−1): ν
̃
= 3449 (m), 2967 (m), 2909 (w), 2873 (m), 2370 (w), 2355
(w), 2338 (m), 2313 (m), 1618 (w), 1599 (w), 1561 (w), 1482 (m),
1410 (w), 1397 (w), 1382 (w), 1362 (w), 1301 (w), 1237 (s), 1168
(m), 1090 (s), 1032 (m), 999 (w), 974 (w), 918 (w), 899 (m), 831
(m), 818 (m), 751 (w), 706 (m), 670 (m), 638 (m), 623 (m), 592
(w), 559 (w), 543 (w), 492 (w), 459 (w). Electrospray ionization mass
spectrometry (ESI(+)-MS): m/z (CH3CN) = 945.1 [M]+. UV−vis
(MeCN): λmax/nm (ε/M−1 cm−1) = 403 (62), 460 (20), 690 (18),
1174 (19). Anal. Calcd for C38H66ClN6Ni2O12PS2 (1046.91): C 43.60,
H 6.35, N 8.03, S 6.13. Found: C 43.97, H 6.34, N 7.78, S 6.27.
Method B (Oxidation with meta-Chloroperoxybenzoic Acid,
mCPBA). To a solution of 2 (95 mg, 0.10 mmol) in dichloromethane
(40 mL) was added with stirring meta-chloroperbenzoic acid (432 mg,
2.50 mmol) at room temperature. The reaction mixture was stirred for
about 24 h after which time solid LiClO4·3H2O (160 mg, 1.00 mmol)
was added. The solution was concentrated under reduced pressure to
produce pale-green crystals, which were filtered, washed with cold
methanol, and dried in air. The crude product was purified by
recrystallization from a mixed acetonitrile/ethanol solution. Yield: 70
mg (67%). The analytical data of this material are identical to those of
compound 6 prepared by method A.
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. The
substitution reaction of [Ni2L(μ-ClO4)]ClO4 (1)13 with [n-
Bu4N]H2PO2 in methanol results in the smooth formation of
the hypophosphito complex [Ni2L(μ-O2PH2)]+, isolable as a
pale-green, microcrystalline perchlorate salt 2 in good yields
(Scheme 2). Complex salt 2 is readily soluble in organic
solvents such as methanol, acetonitrile, and DMF, but virtually
insoluble in water. The corresponding tetraphenylborate salt
[Ni2L(μ-O2PH2)]BPh4 (3) is accessible by salt metathesis of 2
with NaBPh4 in methanol.
Spectroscopic Titration. For the UV−vis−NIR experiments a
4.00 × 10−3 M solution of 2 in MeCN/MeOH (4:1) was aliquoted
into nine samples (0.8 mL) to which increasing equivalents of a 2.56 ×
10−2 M sodium acetate solution were added using an Eppendorf buret.
In that manner solutions containing 0−2 equiv of NaOAc were
obtained and allowed to react for 5 h prior measuring. Spectra were
recorded on a Jasco V-670 UV−vis−NIR spectrophotometer in the
range from 1600 to 190 nm. All spectra were corrected for dilution and
expressed as molar extinction coefficient versus wavelength.
Crystallography. A suitable crystal of compound 3·MeCN was
selected and mounted on the tip of a glass fiber. The intensity data
were collected on a Bruker Smart CCD diffractometer using
Compounds 2 and 3 are thermally stable up to 100 °C in air
in solution (ethylene glycol) as well as in the solid state. This
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phato complex [Ni2L(μ-O2P(OH2)2)]+.12 It is interesting to
note in this respect that a relationship exists between the
basicity of the coligand and the ligand field parameter Δo in
dinuclear nickel(II) complexes supported by the hexamine
dithiophenolate macrocycle L2−.49 For these complexes Δo
decreases with increasing basicity of the coligands. The present
finding that the Δo value for the hypophosphito complex which
Scheme 2. Synthesis of Compounds 2 and 3
−
carries the less basic coligand (pKb (H2PO2 ) = 12.77) is larger
−
than for the dihydrogenphosphato complex (pKb (H2PO4 ) =
stability is quite remarkable given that nickel(II) is readily
reduced by hypophosphite. Moreover, hypophosphito com-
plexes are also very prone to oxidation by air oxygen.7 The
stabilization can be attributed to encapsulation in a molecular
pocket, which has precedence in the literature. Fujita, for
example, has reported on the stabilization of a cyclic trisilanol in
the pocket of a coordination cage.42
11.84) is in excellent agreement with the reported trend.
Cyclic Voltammetry. In order to investigate the redox-
behavior of the [Ni2L(μ-O2PH2)]+ complex, cyclic voltammetry
experiments were carried out. Figure 3 shows the cyclic
voltammogram of a 10−3 molar acetonitrile solution of the
perchlorate salt 2.
All new compounds gave satisfactory elemental analysis and
were further characterized by ESI mass spectrometry, IR, and
UV−vis spectroscopy. The electrospray ionization mass
spectrum (ESI(+)-MS) of a dilute acetonitrile solution of 2
displays a molecular ion peak at m/z = 849.3 in good
agreement with the formulation of the [Ni2L(μ-O2PH2)]+
cation. Infrared data for 2 and 3 and their assignments are
listed in Table 1. IR data for [n-Bu4N]H2PO2 are included for
comparison.
Table 1. Selected IR/cm−1 and UV−vis/nm spectral data for
a
2 and 3
νs(PH2) νas(PH2) νas(PO2) νs(PO2)/
ν1
ν2
[nBu4N]
2345
2308
1198
1052
Figure 3. Cyclic voltammogram of 2 in CH3CN at 298 K.
Experimental conditions: 0.1 M [n-Bu4N]PF6, ca. 1 × 10−3 M sample
concentration, Pt disk working electrode, Ag wire reference electrode,
scan rate 100 mV/s, [Co(Cp2)]PF6 internal reference.
H2PO2
2
3
2332 m 2298 m 1195 m
2334 s
1041 m
1134
1133
671
666
2300 m 1191 s,br 1039 sh
a
m = medium, s = strong, br = broad, sh = shoulder.
The IR spectra show bands arising from the [Ni2L]2+
fragments, the ClO4 or BPh4 counterions, and the H2PO2
Two waves, one at E1 = +0.50 V versus SCE with a peak-
1/2
−
−
−
to-peak separation ΔEp of 0.09 V and one at E2 = +1.24 V
1/2
groups. Complex salt 2 gives rise to two νPH bands, as in other
with ΔEp = 0.12 V, are observed. Coulometric studies reveal
that the first oxidation is a one-electron processes. Controlled-
potential coulometry of [Ni2L(μ-O2PH2)]ClO4 at an applied
potential of +1.0 V versus SCE in CH3CN solution at 273 K
consumed n = 0.97 e−/complex, while re-reduction at 0 V
versus SCE caused transfer of 96% of the charge passed in
oxidation. Furthermore, the CV of the electrogenerated
[Ni2L(μ-O2PH2)]2+ dication 22+ is identical to that of the
starting material.50 The electrochemical properties of [Ni2L(μ-
O2PH2)]+ are comparable to those of the acetato-bridged
complex [Ni2L(μ-O2CMe)]+ which shows two corresponding
waves at E11/2 = 0.56 V and E21/2 = 1.36 V.51 The redox waves
can be assigned to metal-centered (NiIINiII → NiIINiIII) and
ligand-based oxidations (RS−(thiolate) → RS• (thiyl radical))
within the [Ni2L]2+ unit as represented in eqs 1 and 2.51
In the chosen potential window ranging from −2.0 to +2.5 V
versus SCE no redox waves are observed that can be attributed
H2PO2 complexes,43 at 2332 and 2298 cm−1, which can be
−
assigned to the symmetric and antisymmetric νPH2 stretching
modes, respectively. The two intense bands at 1195 and 1041
cm−1, on the other hand, can be attributed to the antisymmetric
and symmetric νPO2 stretching modes.44 The IR spectrum of 3
is very similar to that of 2, and the frequencies of corresponding
−
H2PO2 vibrations are identical (within experimental error).
−
Note that the IR absorptions of the coordinated H2PO2
moieties in 2 and 3 are shifted by as much as 13 cm−1 relative
to the corresponding values of the free H2PO2 ion.45 This
−
frequency shift is in good agreement with coordination of the
−
H2PO2 group.
The electronic absorption spectrum for an acetonitrile
solution of 2 in the 300−1600 nm region reveals two weak
absorption bands at 671 and 1134 nm attributable to spin-
allowed d−d-transitions of a six-coordinate nickel(II) ion
3
3
3
3
−
(assigned as A2g → T1g, A2g → T2g in pure octahedral
to oxidation or reduction of the O2PH2 group. It can be said
symmetry).46,47 For 3, similar absorptions are seen at 666 and
1133 nm. The A1g → T1g(P) transition expected for an
octahedral NiII complex is obscured in both cases by strong
RS−→NiII charge transfer transitions below 450 nm.
−
that the O2PH2 unit is redox inactive in this potential range.
3
3
‐e−
[NiIINiIIL(μ‐O2PH2)]+ HoooI [NiIIINiIIL(μ‐O2PH2)]2+
+e−
‐e−
2+
22+
(1)
The ν1 transition at λmax = 1134 allows us to estimate an
octahedral splitting parameter Δo of ∼8818 cm−1 in [Ni2L(μ-
O2PH2)]+.48 This value should be compared to a similar but
little smaller value Δo = 8795 cm−1 for the dihydrogenphos-
[NiIIINiIIL(μ‐O2PH2)]2+ HoooI [NiIIINiIIL•(μ‐O2PH2)]3+
+e−
22+
23+
(2)
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Inorganic Chemistry
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Description of the Crystal Structure of 3·MeCN. An X-
ray crystallographic analysis using crystals of 3•MeCN grown
from ethanol/acetonitrile was undertaken to unambiguously
characterize the complex. 3•MeCN crystallizes triclinic, space
lengths are common for bioctahedral Ni(II) complexes
supported by L2−.52,53 The average Ni−O bond lengths at
2.030(2) Å are slightly longer than in the dihydrogenphospha-
to-bridged [Ni2L(μ-O2P(OH2))]+ complex indicating that
hypophosphite binds less strongly to the dinuclear [Ni2L]2+
fragment than dihydrogenphosphate,12 a fact that is also
indicated by a smaller Δo value for the hypophosphito complex.
Reactivity of 2. Exchange of Coligands. In a recent ligand
exchange study we have shown that the stability of [Ni2L-
(carboxylato)]+ complexes increases with the basicity of the
carboxylato ligand.49 In other words, a more basic carboxylate
ligand will replace a less basic one. To examine whether this
holds for the hypophosphito complex as well, we performed
ligand exchange experiments of 2 with sodium acetate and
benzoate.
group P1. The crystal structure consists of discrete [Ni L(μ-
̅
2
O2PH2)]+ cations, BPh4− anions, and MeCN solvate molecules.
Figure 4 provides a perspective view of the molecular structure
In an orienting experiment, a reaction of 2 (carrying the
−
weakly basic H2PO2 ion, pKb = 12.77) with sodium acetate
(pKb = 4.75) was followed by UV−vis spectroscopy. The
results of this titration experiment are shown in Figure 5. Upon
addition of increasing amounts of sodium acetate to a solution
of 2 a steady shift of the 3A2g → 3T1g transition band from 671
to 651 nm is observed. Adding further aliquots of NaOAc does
not produce further changes. The absorption at 651 nm can be
attributed to the presence of the acetate-bridged complex
[Ni2L(μ-O2CMe)]ClO4 (4) which has been reported pre-
viously.20,54
Figure 4. Molecular structure of the [Ni2L(μ-O2PH2)]+ cation in
crystals of 3•MeCN. Thermal ellipsoids are drawn at the 50%
probability level. Hydrogen atoms (except for the PH2 group) are
omitted for clarity. Selected bond lengths/Å and angles/deg: Ni(1)−
O(1) 2.026(2), Ni(1)−N(1) 2.224(3), Ni(1)−N(2) 2.150(3), Ni(1)−
N(3) 2.257(3), Ni(1)−S(1) 2.444(1), Ni(1)−S(2) 2.521(1), Ni(2)−
O(2) 2.034(2), Ni(2)−N(4) 2.260(3), Ni(2)−N(5) 2.161(3), Ni(2)−
N(6) 2.226(3), Ni(2)−S(1) 2.4395(11), Ni(2)−S(2) 2.5276(15), P−
O(1) 1.472(2), P−O(2) 1.482(2); Ni(1)−S(1)−Ni(2) 94.99(4),
Ni(1)−S(2)−Ni(2) 90.99(5), O(1)−P−O(2) 120.81(14).
−
Having established that the H2PO2 is readily replaced by
sodium acetate, we performed the same reaction on a
preparative scale. Indeed, when 2 is allowed to react with a
slight excess of sodium acetate in methanol a smooth and
−
−
quantitative exchange of the H2PO2 ion for CH3CO2 takes
place (see Scheme 1), and the known carboxylato complex
[Ni2L(μ-O2CMe)]ClO4 (4)12 (identified by IR spectroscopy
and ESI mass spectrometry, Table 2) can be isolated in almost
quantitative yield. The same is true for a reaction of 2 with
sodium benzoate (pKb = 4.25) which provides [Ni2L(μ-
O2CPh)]ClO4 (5)55 in very good yield. This clearly shows that
the hypophosphito complex 2 is less stable than the carboxylato
complexes 4 and 5.
of the [Ni2L(μ-O2PH2)]+ cation. Selected bond lengths and
angles are given in the figure caption. The hypophosphito
ligand bridges the two six-coordinate NiII atoms in a symmetric
fashion, at a Ni···Ni distance of 3.600(1) Å. This distance is a
typical value for [Ni2L(L′)]n+ complexes coligated with
tetrahedral oxoanions.13 The coordination environment of the
nickel atoms is distorted octahedral, as manifested by large
deviations from the ideal bond angles. The maximum deviation
from the ideal angles is as large as 17.7(1)° (observed for the
O(2)−Ni(2)−N(5) angle). The average Ni−N and Ni−S bond
distances are 2.270(3) and 2.483(1) Å, respectively. Such bond
Oxygenation Reactions. Given that hypophosphite is
readily oxygenated, we sought to transform 2 with H2O2 into
a phosphito or phosphato bridged complex. Treatment of
hypophosphito complex 2 with H2O2 in methanol provides a
Figure 5. Left: Titration of a 10−3 M solution of 2 in acetonitrile/methanol (4:1) with a 10−3 M solution of sodium acetate in MeOH monitored by
UV−vis spectroscopy (590−730 nm region). Right: Plot of the relative shift of the ν2 transition of compound 2 (scaled to a final value of 649 nm for
the acetate-bridged complex 4) versus added equivalents of sodium acetate, with experimental data shown as open circles.
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Inorganic Chemistry
Article
a
Table 2. Selected IR [cm−1] and UV−Vis Data [nm] for 4, 5, and 6
−
−
−
assign
νs(PH2)
νas(PH2)
νas(PO2)
νs(PO2)
νas(RCO2
)
νs(RCO2
)
ν(RSO3
)
ν1
ν2
ν3
4
5
6
1589 s
1569 s
1427 m
1406 m
650
651
690
1131
1123
1174
b
b
2338 m
2313 m
1168 m
1032 m
1237 s
460
a
b
m = medium, s = strong, br = broad. ν3 obscured by strong thiolate→Ni2+ charge transfer transitions.
color change from bright green to pale-green. However, instead
of the anticipated phosphito or phosphate bridged complex,
another hypophosphito complex [Ni2L′(μ-O2PH2)]ClO4 (6)
supported by the hexaamine-diphenylsulfonato ligand (L′)2−
readily identified by UV−vis spectroscopy (by the lack of
intense thiolate→Ni2+ transitions) and IR spectroscopy (by IR
ligand field may act on the 3A2g ground state to produce a zero-
field splitting which may be of the same order of magnitude as
J.60,61 The values of g and D were assumed to be identical for
both nickel ions.
2
2
̂
̂
H = −2JS1S2 + ∑ [D(Siz − 1/3Si(Si + 1)) + g μ BτSiτ]
i
−
−
B
i
bands for the RSO3 and the H2PO2 ion) has formed (see
Table 2). Thus, instead of an oxygenation of the hypophosphito
coligand, an S-oxygenation of the supporting N6S2 ligand to the
known N6(SO3)2 (hexaamine-diphenylsulfonato) ligand (L′)2−
has taken place.56 The same compound 6 can also be
synthesized by using metachloroperbenzoic acid (MCPBA) as
i=1
(t = x, y, z)
(3)
By taking into account the zero-field splitting and temper-
ature independent paramagnetism,62 a good fit of the
experimental data, shown in Figure 6 as a solid line, was
possible over the full temperature range, yielding J = +22 cm−1,
g = 2.17, TIP = 1.40 × 10−4 cm3 mol−1, and D = −8.12 cm−1.
However, the value of the zero-field splitting parameter should
be taken as indicative rather than definite, because temperature
dependent magnetic susceptibility measurements are not very
appropriate for the determination of D.63,64 This is
corroborated by a two-dimensional projection of the relative
error surface for varying J and D (Figure S1). Instead of one
global minimum the error surface exhibits two minima with the
same relative error corresponding to the combinations D =
−8.12 cm−1, J = +22 cm−1 and D = +8.12 cm−1, J = +22 cm−1.65
Nevertheless, J is not influenced markedly by the value of D and
represents an accurate measure of the magnetic coupling in this
complex. The S = 2 ground state for 3 is confirmed by field
dependent magnetization measurements (see Supporting
Information).
an oxidant.57 The apparent resistance of the H2PO2 ion
−
toward oxidation is attributed to its steric protection by the
binding pocket of the [Ni2L]2+ fragment.
Magnetic Properties. Figure 6 displays the temperature
dependence of the molar magnetic susceptibility (per dinuclear
It has been noted previously that there exists a relationship
between the sign and magnitude of the coupling parameter and
the Ni−S−Ni angle in bioctahedral dinickel(II) complexes
supported by L2−.53,66 The coupling is antiferromagnetic in
nature if the Ni−S−Ni angle deviates by more than ≈ 10°
from 90°. For angles within the 90 10° range a ferromagnetic
interaction is observed. The observation of a ferromagnetic
exchange interaction in 3, for which the average Ni−S−Ni
angles is in the 90 10° range, is in good agreement with the
reported trend.
Figure 6. Temperature dependence of χMT for 3 (per dinuclear
complex). The full line represents the best theoretical fit to eq 3.
Experimental and calculated values are provided as Supporting
Information.
In complex 3 two coupling pathways are possible, one
through the thiolato and one through the hypophosphito
coligand. In order to establish the predominant coupling path,
we have compared some [Ni2L(L′)]+ complexes with an S = 2
ground state. Table 3 lists the J and g values of the selected
examples, along with their Ni···Ni distances and Ni−S−Ni
angles. As can be seen, the Ni···Ni distances and the Ni−S−Ni
angles lie within very narrow ranges, and the same holds for the
J values as well. The similar J values imply that the magnetic
exchange interactions are predominantly propagated via the
two thiophenolato bridges (in case that the Ni−S−Ni angles
are close to 90°). In view of longer coupling pathways through
the Ni−L′−Ni bridges, this is not surprising.
complex), in the form of a χMT versus T plot for 3. The value of
χMT increases from 2.72 cm3 K mol−1 (4.67 μB) at 330 K to a
maximum value of 3.50 cm3 K mol−1 (5.29 μB) at 30 K, and
then decreases to 2.64 cm3 K mol−1 (4.59 μB) at 2 K. This
behavior indicates an intramolecular ferromagnetic exchange
interaction between the two NiII ions that leads to an S = 2
ground state of the hypophosphito complex. The decrease in
χMT below 20 K can be attributed to zero-field splitting of NiII
and/or saturation effects.
The temperature dependence of the magnetic data of
complex 3 was simulated using the appropriate spin-
Hamiltonian (eq 3)58 which contains a Zeeman and a zero-
field splitting term applying a full-matrix diagonalization
approach.59 The introduction of a D parameter is appropriate
for octahedral NiII, since the noncubic components of the
DFT Calculations. We performed DFT calculations to get
more insights into the magnetic coupling in the [Ni2L(μ-
O2PH2)]+ complex. With regards to the type of the exchange
interaction, our calculations predict a ferromagnetic coupling in
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Inorganic Chemistry
Article
the overall HLG. In essence all states close to EF are dominated
by Ni d-states. This would also be in good qualitative
agreement with the cyclic voltammetry experiments described
above where the first oxidation of [Ni2L(μ-O2PH2)]+ is metal-
centered (Ni2+ → Ni3+) in nature.
Table 3. Magnetic Properties and Metrical Data of Selected
[Ni2L(L′)]+ Complexes Supported by the N6S2 Macrocycle
L2−
a
J
Ni···Ni
[Å]
Ni−S−Ni
complex
[cm−1
]
g
[deg]
ref
this
[Ni2L(μ-O2PH2)]+
(3)
+22.0
+21.7
+18.4
2.17
3.600(1)
3.483(1)
3.491(1)
93.00(4)
89.65(4)
89.56(4)
CONCLUDING REMARKS
work
■
[Ni2L(μ-OAc)]+
(4)
2.14
2.19
55
The main findings of the present work can be summarized as
follows: (i) The dinuclear hypophosphito-bridged Ni(II)
complex [Ni2L(μ-O2PH2)]+ supported by a macrocyclic
hexamine-dithiophenolate ligand is readily prepared, and can
be isolated as an air-stable perchlorate [Ni2L(μ-O2PH2)]ClO4
(2) or tetraphenylborate [Ni2L(μ-O2PH2)]BPh4 (3) salt. (ii)
Complex salts 2 and 3 were found to be thermally stable both
in solution as well as in the solid state, which contrasts the
behavior of other nickel hypophosphito systems, which are
readily reduced by hypophosphite. (iii) The stabilization is
believed to be a consequence of the steric protection offered by
the supporting macrocycle. (iv) The bisoctahedral [Ni2L(μ-
O2PH2)]+ cation has a N3Ni(μ1,3-O2PH2)(μ-S)2NiN3 core
structure with the hypophosphito ligand attached to the two
NiII ions in a μ1,3-bridging mode. (v) The hypophosphito ligand
can be replaced by carboxylates, in agreement with a higher
affinity of the [Ni2L]2+ dication for more basic oxoanions. (vi)
Treatment of [Ni2L(μ-O2PH2)]ClO4 with H2O2 or MCPBA
results in the oxidation of the bridging thiolato to sulfonato
groups. The hypophosphito group is not oxidized under these
conditions again most likely due to the steric protection offered
by the supporting N6S2 macrocycle. (vii) The O2PH2− unit in 2
is redox inactive in a potential window from −2.0 to +2.5 V
versus SCE. (viii) Finally, the spins of the nickel(II) (Si = 1
ions) in 3 are weakly ferromagnetically coupled [J = 23 cm−1,
(H = −2JS1S2)] to produce an S = 2 ground state.
[Ni2L(μ-OBz)]+
(5)
55
[Ni2L(μ-H2BH2)]+
+27.0
+20.0
2.09
2.20
3.458(1)
3.425(1)
89.87(4)
87.12(3)
67
68
[Ni2L(μ2,3
-
b
N4CCH3)]+
a
Parameters resultant from least-squares fit to the χMT data under the
spin Hamiltonian in [eq 3], J = coupling constant (H = −2JS1S2), g =
b
−
g-value. N4CCH3 = 5-methyl-tetrazolato.
agreement with the experimental data and regardless of the
applied exchange correlation functional. However, the DFT-
(PBE) calculation (J = 38 cm−1) overestimates the exchange
coupling, an effect which is well-known since standard DFT
exchange correlation functionals do not localize the d-states
sufficiently.69,70 The B3LYP density functional somewhat
corrects for this DFT failure by mixing Hartree−Fock exact
exchange into the functional. Therefore, the exchange coupling
evaluated from the B3LYP calculations is often in better
agreement with the experimental ones.71 Accordingly, the
calculated B3LYP exchange interaction (J = 16 cm−1) is closer
to the experimental one.
To gain a more detailed insight into the magnetic coupling
an additional DFT(PBE) calculation without the bridging
hypophosphito ligand was performed. The overall ferromag-
netic coupling is preserved upon this removal; however, the
strength of the coupling is reduced by a factor of 2 resulting in J
= 15 cm−1. The lowering of the J value indicates that the
ASSOCIATED CONTENT
* Supporting Information
■
S
−
H2PO2 ion provides a coupling path and contributes to the
X-ray crystallographic data of 3•MeCN in CIF format.
Experimental and calculated susceptibility data, experimental
and calculated magnetization data and DFT calculation for 3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
ferromagnetic exchange coupling. Figure S3a shows a
perspective view of a molecular orbital presenting a distinctive
−
pathway across the H2PO2 ion in 3. According to the
Anderson−Goodenough−Kanamori rules72−75 another ferro-
magnetic coupling path via the sulfur atoms should be available
since the Ni−S−Ni angles in 3 are close to 90° (Ni1−S1−Ni2
94.99(4)°, Ni1−S2−Ni2 90.99(5)°). Indeed, such a pathway
across the sulfur atoms is available as indicated by Figure S3b.
This result is in good agreement with the reported trend.53
Another, more quantitative, point of view is based on the
Hay−Thibeault−Hoffmann model,76 where the square of the
energy difference (Δe)2 of the unoccupied magnetic orbitals at
the metal centers is of interest. In the case of 3 we are
confronted with a very small energy difference of (Δe)2 =
0.0173 eV2 of the unoccupied Ni orbitals. This value lies within
the range of (Δe)2 = 0.0−0.027 eV2 typical for [Ni2L(L′)]+
complexes for which a ferromagnetic exchange interaction is
expected.53
AUTHOR INFORMATION
Corresponding Author
*E-mail: b.kersting@uni-leipzig.de. Fax: +49/(0)341-97-36199.
■
Present Address
§Institute of Chemistry, Academy of Sciences of Moldova
Academiei str. 3, MD-2028, Chisinau, Republic of Moldova.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Deutsche Forschungsgemein-
schaft DFG-FOR 1154 “Towards molecular spintronics” and
the University of Leipzig. C.L. (ESF:080945373) and J.L. thank
the SAB for an ESF grant. Special thanks go to the Center for
Information Services and High Performance Computing at the
TU Dresden for computational time and support.
Figure S4 shows the density of states close to the Fermi Level
EF, which is defined in the middle of the HOMO−LUMO gap
(EF = (LUMO−HOMO)/2). One can immediately see that the
HOMO can be attributed to majority spins while the LUMO
consists of minority spins. The HOMO−LUMO gap (HLG)
between these two states is 1.27 eV. The HLG for majority
spins is considerably larger (2.82 eV), and the HLG for
minority spins (1.82 eV) is in between the majority HLG and
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