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New Sulfur- and Selenium-Bridged Copper Clusters; Ab Initio Calculations on [Cu2nSen(PH3)m] Clusters.

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New Sulfur- and Selenium-Bridged
Copper Clusters; Ab Initio Calculations
on [Cu,,Se,(PH,),J Clusters **
Stefanie D e h n e n , A n s g a r Schiifer, Dieter Fenske,* and
R e i n h a r t Ahlrichs
Copper(r) chloride reacts with tertiary phosphanes and
SefSiMe,), with the elimination of SiMe,Cl to give seleniumbridged copper cluster complexes.['] Hitherto only metal-rich
copper clusters have been observed and characterized as reaction products for these reactions. Examples of these include:"]
ligands.['] The calculations were carried out with the TURBOMOLE"] program at the MP2 approximation,[" which had
proved successful for the above study, whereas the SCF method
had proved to be insufficient.
Several important structure principles shall be outlined by
using clusters 5-8 (Fig. 1 ) as examples; each of these form local
minima on the energy hypersurface. More accurate calculations
were also carried out for 5 at the coupled cluster level
(CCSD(T)), from which errors in the Cu-Cu ( t - 5 pm) and
Cu-Se ( i 3 pm) bond lengths of the MP2 structures can be
In contrast, it has not been possible to isolate intermediates of
these reactions, as has been achieved in the synthesis of tellurium-bridged copper clusters.[']
In 1991 I. G . Dance et al. reported the existence of smaller
copper chalcogenide cluster ions, such as [Cu, ,Eel-, [Cu,,E,]-,
and [Cu,,E,,J, which were produced after laser ablation ofthe
elements or of the binary chalcogenides, and subsequently could
be detected by mass spectrometry.I3I This encouraged us to
search for synthetic methods for ligand-protected copper clusters of this type.
In order to synthesize these intermediates as well as the sulfurbridged species still missing in the series of chalcogenide-bridged
copper clusters, copper(1) acetate was treated with PR,R' (R,
R' = Ph. Et) and E(SiMe,), (E = S, Se) in diethyl ether or THF.
In this way compounds 1-4 were isolated and characterized by
X-ray crystal structure analyses (see Figs. 3 and 4) .[4. 51
Fig. 1. Structures of 5-8 resulting from MP? calculations. Selected bond lengths
[pm] (error k 5 p m ) and angles [ I : 5: C u l - C u l ~221.6. C u l Cu2 260.2. C u 2 Cu2'261.2. C u l - S e 237. Cu2-Se 222.8. - 6 : C u l Cul'244.4, Cul-Cu2 251.
Cu 1 -Cu2323.9. Cu2-Cu2'260.4. Cu 1 Se'237.7. C u l -Se244.3. Cu2-Se222.7,
C u t P I 218.3, C u 2 - P Z 216; Se-Cu2-P2 172. - 7: C u l C u l ' 262.6, C u l Cul"275.4. Cul-CuZ 260.1, C u l " C u 2 223.8, C u l - S e 245.2, Cu2-Se 233.1. 8 : C u l -Cul'280.4. Cul-Cui"260.5. Cu I~ Cu2 271.2, C u l " - C u l 234.7. Cu 1 Se 232.2. Cu2-Se 246.5. Cu2-P 219.
Compounds 2, 3. and 4 are the first sulfur-bridged copper
clusters which could be prepared in this way and characterized.
Compounds 1 and 4 precipitate in the form of red crystals from
dark brown solutions; 2 forms red and 3 forms violet crystals,
which were each isolated from colorless reaction solutions. The
IR spectra of the compounds are not very informative, since
they only contain characteristic vibrations for the phosphane
After the synthesis of 1. a b initio calculations for [Cu,,Se,]
were carried out parallel to and independent of the crystal structure analysis of 1. The structural features and energetics of such
molecules had been investigated previously as part of a systematic study of small [Cu2Se],,clusters with and without phosphane
[*] Prof. Dr. D. PenTke. DiplLChem. S. Dehnen, Dipl.-Chcm. A. Schiifer.
Prof. Dr. R. Ahlrichc
lnstitut fur Anorganische Cheinic der Universitit
Engcrserstraaae, Crebiude-Nr. 30.45, D-76131 Karlsruhe (FRG)
Telcfilx. Int. code + (721)661-921
This uork
wit5 supported
b) the Dcutsche ~orschungsgeincinscliaft(SFB 195)
For at least three, but mostly four Cu-Se contacts per selenium atom, the structure with the maximum number of short
Cu-Cu contacts ( < 260 pm) is always preferred. Generally. this
leads to structures with relatively high symmetry. Accordingly.
a distorted Cu, tetrahedron exists in 5 in which two Cu, faces
are capped by p,-selenido ligands. In contrast, in 7 a highly
distorted cube of copper atoms is observed, whose two seleniumfree faces are deformed to rhombuses that are rotated by 90'
relative to each other. The Cu-Cu interaction exhibits marginal
covalent contributions and is best considered as a dispersion
interaction of the d" systems.[91
The PH, ligands in 6 and 8 coordinate to exposed sites on the
cluster framework and favor- as far as possible-a linear SeCu-P arrangement (the Se-Cu2-P2 angle in 6 is 172'): this tendency is typical for the coinage metals in the oxidation state
For increased loading with Se ligands. trigonal-planar
coordination at the Cu centers is preferred (in 8 P projects 21.5"
above the Se -Cu2 - Se plane). The rules found for smaller molecules, regarding the choice of the most stable arrangement of
copper and selenium atoms, support the structure of 9 (Fig. 2).
Other considered structures always gave higher energy. Compound 9 has 0, symmetry and can be described as a Cu,, cubooctahedron with Se-capped square faces or as a Se, octahedron whose edges are linked by Cu atoms.
ly.[*]The bond energy of the naked clusters increases with growing cluster size; thus, these clusters are not thermodynamically
stable systems.
From the calculated total energies (MP2) of 6 , 8 , and 10, the
following bond energies of the Cu-P bonds result: 103, 98, and
50 kJmol-'. The Cu-P bond becomes weaker as the size of the
cluster increases, since the copper atoms are then loaded with
selenium neighbors, and thus, the ideal (linear) Se-Cu-P arrangements are no longer formed. This also explains why the
PR, ligands preferably bind to the exposed copper atoms. The
low bond energy of the Cu-P bond (50 kJmol-') in 10 clarifies
why 1 must be prepared with cooling.
The structural buildup of 1.2, and 3 is the same. The molecular structures of these compounds in the crystal is exemplified in
Figure 3, which shows the structure of 1 . Compounds 1, 2, and
Fig. 2. Structures of 9 and 10, resulting from MP2 calculations. Selected bond
lengths [pm] (error k5 pm): 9: Cu-Cu' 252, Cu-Se 233.3, 10: C u l - C u l ' 275.2.
Cul-Cu2 256.5, Cu2-CuZ' 283.4. C u l - S e l 234.5. C u t - S e 2 242.1, C u 2 Se2 227.6. Cu 1-P 232.6.
For the calculation of the structure of the ligand-clad cluster
framework of 9. PH, was used as a model ligand and the highest
possible symmetry, D,,,.
was assumed for the resulting molecule
10. This considerably reduced the effort required for the calculation. The structure of 10 (Fig. 2) determined in this way is in
agreement with (within the limits of the methods) the molecular
structure of 1 (cf. Fig. 3)[,] determined by crystal structure analysis. The calculated Cu-Se distances in 10 lie in the range of
measured values of 1 o r slightly above ( < 2 pm). The calculated
Cu-P bond is at least 3 pm longer than that found in 1, which
can be attributed to the fact that substituted phosphanes are
stronger donors than PH,. Slightly larger deviations occur for
several Cu-Cu distances; the greatest difference ( 1 1 pm) between the calculated and measured values is found for Cu 1Cu2 (see Fig. 3 ) . Here, however, very soft modes are observed-- in the small clusters they typically lie at 100 cm-'; as a
result packing effects might play a role in the crystal.
The PH, ligands lead to a distortion of the Cu,, framework
in 9, in such a way that two face-shared Cu, antiprisms are
formed in 10. The copper atoms bearing phosphane ligands
(Cu 1 -Cu4 in 1) in 10 and 1 are positioned further away from
the center of the cluster than in 9 and adopt trigonal-planar
coordination. In contrast, the four remaining copper atoms
(Cu5. C u 6 and inverted positions in 1) are shifted further towards the center of the cluster, retaining almost linear Se-Cu-Se
arrangements (the angle is about 167" in 10); as a result the
coordination of additional ligands to these atoms is prevented.
The stabilization energy Enof the naked clusters 5,7, and 9 is
calculated to be -160, -302, and -427 kJmol-', respective-
Fig. 3. Structure of [Cu,,Se,(PEtPh,),] 1 in the crystal (without ethyl and phenyl
groups. C u - - C udistances up to 318 pm are drawn). Selected bond lcngths [pm] and
angles ["I (error k0.9 pm and kO.3'. respectively): Cu(1.2,3.4)-Cu 257.2 192.8.
Cu5-Cu(6,6)296.2aiid 286.9. S e l - C u 232.9-233.9. SeZ-Cu220 245, Se3 Cu
216.5-239.3. P-Cu 226.2-229.1; Scl-Cu-Se 143.4 147.4, CuI-Sc2'-Cu3' 123.2,
Cu 2-Se 3'-Cu4' 127.9, Se 1-Cu-P 11 5.7- 120.9, Cu 3-Cu S-Cu 3' 11 3.2.
3 have an inversion center. The copper part of the framework
consists of two face-shared, slightly distorted square antiprisms.
This rare structural fragment exists, however. in a highly
deformed form, for example, in [Rh,,C2(C0),,]2-.
[Rh,,C2(C0),,]3-, and [Rh,2C,(C0),,]4-,[51 The distances of
the copper atoms from the inversion center range from 205.4300 pm in 1, 206.4-295.9 pm in 2, and 203.8-310 pm in 3. The
atoms of the middle Cu, faces (Cu 5, Cu 5', Cu 6, Cu 6') are each
closer to the center than those of the basal faces ( C u l -Cu4).
Thus, the copper part of the framework cannot be described as
a distorted cubooctahedron; cubooctrahedral cluster compounds or structural fragments always have almost identical
distances between the apecies of the polyhedron and the center.
One example of this is the framework of copper atoms in
[ c uI ,(PPh),(PPh 3)61 .['I
[*] The calculated total energies (MP2) of the naked clusters 5 . 7. and 9 uainp the
basis set given in [6] are -220.962573, -441.141384. dnd -663.497772 au.
respectively; 1 a u (atomic unit) = 2625.5 kJrno1-I. For the ligand-clad clusters
6, 8, and 10 values of -253,693704. -474.865664. and -728.799384 au are
The eight phosphane ligands in 1, 2, and 3 coordinate to the
eight copper atoms of the two basal faces. The sulfur and selenium atoms always act as ,u,-bridging ligands. Together they form
a slightly distorted octahedron. The copper atoms of the outer
faces (Cu 1 -Cu4) are each bound to two chalcogen atoms and
one phosphorus atom (E-Cu-E = 142- 147", E-Cu-P = 112122', and 95-105'). The copper atoms of the middle face are
each coordinated by two chalcogen atoms, leading to an almost
linear E-Cu-E arrangement (1 72- 176"). In the equatorial face
of the chalcogen octahedron an eight-membered Cu,E, metallacycle (Cu5. Cu6, E2, E 3 and inverted positions) is found.
Cu,R, metallacycles without additional coordination are
known for R = CH,SiMe, o r R = O-rert-butyl.["l The ability
of chalcogen atoms to adopt higher coordination enables the
formation of the three-dimensional structures for the cluster
complexes presented here.
In addition to differences in bond lengths, compounds I , 2,
and 3 show different degrees of tetragonal distortion. In this
way the molecules (without taking the organic groups into consideration) do not achieve D,,,symmetry, but only D,, symmetry. The Cu-Cu distances in all of the compounds are too large
(255-354 pm) to be considered as bonding interactions. The
Cu-Cu and the Cu-E distances found in 1-3 are of the same
order of magnitude as those in binary chalcogenides. This was
also established for other copper-selenide clusters.[']
The molecular structure of the cluster complex 4 (Fig. 4) can
now be derived in a simple way from the structures of 1. 2, and
3: in 4, two Cu,, fragments were formally linked through one of
the Cu, faces previously described as basal faces. The sulfur
atoms which are arranged above these faces in 1, 2, and 3 and
the eight phosphane molecules bound to the corresponding copper atoms are no longer present after this formal cluster condensation. Instead the new, almost square Cu, face is extended;
thus, the framework of copper atoms in 4 forms an arrangement
of four square antiprisms linked through three Cu, faces. An
inversion center is also present in 4.
There are numerous examples of the condensation of cluster
fragments to larger units. Face-shared octahedra exist, for example, in the species [ C S , , ~ ,and
] [Rb,0,].[12. l 3 I described by
A. Simon; examples for the formation of strands of octahedra
by face-sharing include the Chevrel phases [Mo,,Se,,12-,
[ M o , , S ~ , , ] ~ - ,[Mo2,Se,,l6-, and [ M o , , S ~ , , ] * - . [ ~14.~ ~1 5 ] Cluster complexes which exhibit this building principle are also known
for cobalt and nickel: [CogSe,,(PPh,),]. [Ni,,Se,,(PPh,),], and
[Ni,zSe,,(PEt,)6]."61 Ecliptic stacking oftrigonal prisms through
triangular faces has been described for [Pt, ,(CO),,]3 .[51
The linkage of more than three tetragonal antiprisms is, however, new in cluster chemistry. Besides icosahedral (e.g. [Ni,,As,(CO)l,(CH3)z]2-) and anticubooctahedral structures (e.g.
[PtRh,,(CO),,]"~) as well as polyhedra consisting of three faceshared octahedra (e.g. [Ir,z(CO)26]2-)also many, far less symmetrical arrangements (e.g. [Co,Ni,C(CO),,]
and [Os, I CuC(CO),,(CH,CN)]-)[51 of twelve metal atoms in cluster compounds have been observed. For the few known metal clusters
with 20 metal atoms so far no uniform structural principle has
been formulated.
If it is assumed that in 1-4 the chalcogenido ligands exist as
SeZ- or S", the copper fragments of the compounds carry the
formal charges Cuf;' and Cu:,"' . Consequently, the copper
atom in the cluster complexes can be assigned the formal oxidation state + I ( d t oconfiguration). This supports the assumption
that metal-metal bonding can be ruled out in these systems, and
that at most weak interactions exist between the copper atoms.
There are. however, several cases in which d" -dIo interactions
contribute to metal bonding.["* 171
Compounds 1-4 are possibly intermediates In the synthesis of
larger cluster compounds. This is indicated first by the dark
brown color of the reaction mixtures from which the red crystals
of 1 and 4 were isolated, and second the compounds are more
readily decomposed than known large cluster complexes. There
is still a lack of concrete evidence for the formation of larger
cluster complexes in the colorless reaction mixtures from which
2 and 3 were isolated.
E.xperimmtal Procedure
I : PEtPh, (1.61 mL. 6 mmol) was added to a suspension of CuAc (0.37 g, 3 mmol)
in diethyl ether (25 mL), which resulted in the dissolution of the CuAc. The reaction
mixture was cooled to -65 'C and treated with (0.37 mL. 1.5 mmol) Se(SiMe,),.
The solution was allowed to warm to room temperature over 4 d. during which time
it had turned dark brown and red needles of I had crystallized out (yield 5%).
2 : PEtPh, ( I .61 mL. 6 mmol) was added to a suspension of CuAc (0.37 g. 3 mmol)
in diethyl ether (25 mL), which resulted in the dissolution of the CuAc. S(SiMe,),
(0.31 mL. 1.5 mmol) was added to the solution at - 78 C. and the reaction mixture
was subsequently allowed to warm to - 20 -C over 4 d . Cherry red rhombuses of 2
crystallized from the colorless solution (yield 100%).
Fig. 4. Structure of [Cu,,S,,(PPh,)J 4 in the crystal (without phenyl groups,
Cu Cu distances up to 354 pm are drawn) Selected bond lengths [pm] and angles
[ ] ( e r r o r f 0 . 9 pm and f 0 . 4 - , respectively): Cu(1.2.3,4)-Cu 264.1 290. Cu(9.10)Cu 261.1-354.4, S 1 -Cu 224.5-226.1. S2-Cu 214.2- 235.8. S3-Cu 215.7-235.1.
S4-Cu 215.7-234.8. S5-Cu 214.9-235.2, P -Cu 227.3-229.6; S I-Cu-S 140.2152, Cul-S2-Cu9 140.1. Cu2-S3-CuIO 138.9, Cu3-S4-Cu9' 140.6, Cu4-S5-CulO
140, Sl-Cu-P 117.2-120.2. C u l - C u 5 - C u 9 103.5. Cul-Cu8-Cu9 105, Cu2-Cu5C u l O 104.8, Cu2-Cu6-CulO 103.2, Cu3-Cu6-Cu9' 105.4. Cu3-Cu7-Cu9' 102.3.
Cu4-Cu7-CulO 105.1. Cu4-CuX-CulO 102.9.
VCH V F r I u g ~ ~ r ~ r l l s ~
mlhi H
u f fDo-69451Wcrnk~im I994
3: PEt, (1.18 mL. 8 mmol) was added to a suspension of CuAc (0.25 g, 2 mmol) in
diethyl ether (40 mL). which resulted in the dissolution of the CuAc. S(SiMe,),
(0.21 mL, 1 mmol) was added to the solution at -78 ' C , and the reaction mixture
was subsequently allowed to warm to -20 C over 4 d. In this way violet square
crystals of 3 were obtained in 82% yield.
4: PPh, (1.31 g. 5 mmol) was added to a suspension of CuAc (0.12 g. 1 mmol) in
T H F (20 mL), which resulted in the dissolution of the CuAc. Subsequently.
S(SiMe,), (0.10 mL, 0 5 mmol) was added. which caused the reaction mixture to
turn dark brown. The mixture was cooled to -40 ' C , and after 7 d a powdery
precipitate was filtered off. Red octahedra of 4 crystallized out after a further 6 d at
Received: October 25. 1993 [Z6448IE]
German ~ e r s i o n A
: N ~ I I Clirin.
1994, 106. 786
0570-OR33 94 0707-0748 $ 10 OOf 75 0
Angebi Clicin Inr Ed Engl 1994, 33 Vo 7
[I] D. Fenske. H. Krautscheid. S. Balter, Angekv. Chem. 1990, 102, 799-801;
Angrii.. Chem. lnt. Ed. Engl. 1990, 29, 796-798 D. Fenske. H. Krautscheid,
ihrrl. 1990. 102. 1513-1515 and 1990, 29. 1452-1454: H. Krautscheid. D.
Fenske. G. Baum. M. Semmelmann. ibid. 1993, 105.1364-1367 and 1993.32,
1303 1305.
[2] D. Fenske. J.-C. Steck. Angew. Chen7. 1993, 105. 254- 257: Arigew. Chem. In/.
Ed. EngI. 1993. 32 238-242.
[3] 1. H. El Nakat. I. G. Dance, K. J. Fisher, G. 5. Willet, Inorg. c'hem. 1991. 30,
[4] X-ra! structure analyses: STOE IPDS: Mo,,, 213 K ; data collection and refincment. I L I =1380.3(10). b =1452.6(12), r=1733.9(12)pm, a =107.5(4),
p=101.11(4). y=l12.18(4)'. V=28X0(4)x1Ohpm3. PT (no. 2), Z=1.
p(Mok,) = 11.23 cm-'. 28,,, = 52 , 20613 reflections. 10232 of which are independent, 2245 with / > 6 u ( / ) . 622 parameters (Se, Cu, P. C anisotropicically
refined). R = 0.082, As a result of the very low scattering power of the crystals
so far no better data set has been obtained. An isotropic refinement of the C
atoms leads to R = 0.093. - 2: a =1377.0(11). h =1441.0(10),
[j =101.52(4),
V = 3852(4) x 10" pm3, PT (no. 2). Z = 1, p(MoKJ = 24.05 cm-',
20,,,, = 56 ; 22531 reflections, of which 12363 are independent, 5096 wlth
/ I 60(/). 622 parameters (S, Cu. P, C anisotroprcically refined), R = 0.063. 3: o =1203.4(12). b =1254.5(10). c =1492.8(10) pm. a = 66.03(4).
/)=68.14(7), ;'=78.4(6), V=1908(3)x10bpm3. P7 (no. 2), 2=1.
p(MokJ = 35.57 c m - l , 20,,, = 61 '; 32712 reflections, ofwhich 8370 are independeiit. 5387 with />60(/), 242 parameters (S. Cu, P anisotropicically refined: the ethyl groups are partly double disordered, their atomic positions
were refined isotropically), R = 0.072. - 4.8THF: a = 2526(2), h = 2798.1(8),
r = 2899(2) pm,
V = 20489(22) x l o b pm3, Pbra (no. 61). Z = 4.
!<(Mo&J = 21.79 c m - ' . 2OmaX= 42'. 74443 reflections, ofwhich 10857 are independent. 3249 with /> 6u(/). 396 parameters (S, Cu, P anisotropicically
refined: phenyl rings refined as rigid groups). R = 0.094. Four solvent molecules s e r e located per asymmetric unit. Further details of the crystal structure
investigations may be obtained from the Fachinformationszentrum Karlsruhe,
D-76344 Eggenstein-Leopoldshafen (FRG) on quoting the depository number
[5] D. F- Shriver, H. D. Kaesz, R. D. Adams. The Cbemrsrrj of Metal Cluster
Con7plcwr. VCH. New York. 1990.
[6] A. Schifer. C. Huber. J. Gauss, R. Ahlrichs. Theor. Chim. Acta 1993, 87, 29:
A. Schiifer. R. Ahlrichs. unpublished.
[7] R. Ahlrichs, M . Bir, M. Hiser. H. Horn, C. Kolmel, Chem. P h w Lett. 1989,
162. 165: F Haase. R. Ahlrichs. J Comput. Chem. 1993, 14. 907.
181 MP2 denotes the treatment of electron correlation in the second order of
perturbation theory starting from a Hartree-Fock wave function: C. Msller,
M . S. Plesset. P/I!S. Rev. 1934, 46.618. Relativistically corrected pseudopotentiak with 18 core electrons for Cu and 28 core electrons for Se were used for
the calculations; M. M. Hurley, L. F. Pacios, P. A. Christiansen, R. B. Ross,
W. C . Ermler. J Cbrrn. Phjs. 1986, R4, 6840. The basis sets Cu: (3s2p6d)/
[2s2p?d] and Se: (3~3pld);[2~2pld]
were optimized for the atoms a t the SCF
level. polarization functions for Cu,Se at the MP2 level [7]. A pseudopotential
with 10 core electrons was used for P;W. R. Wadt, P. J. Hay, J Chem. Pl73.s.
1985.82.284.The associated basis set was further optimized (changes 5 % ) and
supplemented with a set of d functions (vd = 0.45). A 3s basis set was optimized
for H and contracted to one function.
[9] C Kiilmel. R. Ahlrichs, J Phys. Chern. 1990, 94. 5536.
[lo] A. Grohmann, J. Riede, H. Schmidbaur. Narure 1990.345, 140.
[ l l ] F. A. Cotton. G. Wilkinson, Advunced lnurgunic Chemistry, 5. edition, Wiley,
Neu York. 1988.
[I?] A. Simon. Angew. Chem. 1981. 93, 23-44: Angetr. Chem. Inr. Ed. Engl. 1981,
20, 1-22: ibid. 1988, 100, 163-188 and 1988. 27. 159-183.
[13] A Simon, J Solid Sture CAem. 1985, 57. 2.
[I41 0. Bars. J. Guilleric, D. Grandjean. J SolidSrate Chem. 1973,6. 6: R. Chevrel,
P.Gougeon. M. Potel. M . Sergent, hid. 1985,57. 25: R . Chevrel. M. Sergent,
B. Seeher. 0. Fischer, A. Gruttner, K. Yvon, M a w . Re.s. BUN. 1979. 14, 567;
A. Gruttner, K. Yvon. R. Chevral. M. Potel, M. Sergent, B. Seeber, Acta
CryruiloRr. S K ~ .5 1979. 35. 285, M . Potel. R. Chevrel, M. Decroux. 0.
Fischer, C R . Hehd. S6ances Acrid. Sci. Ser. C 1979, 288, 429.
1151 W. Honle. H . G. von Schnering, A. Lipka, K. Yvon, J Less-Common M e t .
1980. 71. 135.
[I61 D. Fenske. J. Ohmer. J. Hachgenei. K. Meraweiler. Angrw. Chem. 1988, 100.
1300-1320; Arigew. Chem. Inr. Ed. Engl. 1988, 27, 1277-1296.
1171 F. A . Cotton, X. Feng, M. Matusz. R. Poli. J Am. Chem. Soc. 1988,110,7077:
R. Mason. D. M. P. Mingos. J ffrgunomet. Chem. 1973, 50. 53: P. K. Mehrotra. R . Hoffmann. Inorg. Chem. 1978. 17, 2187; K . M. Merz. R. Hoffmann,
ihid. 1988, 27. 2120.
Borophosphates-A Neglected Class of
Compounds: Crystal Structures of M"(BP0,J
(M"= Ca, Sr) and Ba3[BP3012]**
Rudiger Kniep," Giiller Gozel, Brigitte Eisenmann,
Caroline Rohr, Matthias Asbrand, and Meral Kizilyalli
Given all the current activity in the field of microporous solids,
it is remarkable that intermediate phases of the MiO/M"OB,O,-P,O,(-H,O) systems (M' = alkali metal, M" = alkaline
earth metal) have been "forgotten". Bauer". (1965/1966) reported the synthesis and X-ray diffraction patterns of the isostructural (hexagonal) phases M"[BPO,] (M" = Ca, Sr, Ba).
Ramamoorthy and Rockettc31(1974) confirmed the existence of
Ca[BPO,] from phase equilibria studies on the system CaOB,O,-P,O, at 900°C. Liebertz and St%hrf4I(1982) reported on
the compound Mg,[BPO,] (dimorphous; isostructural with ci-@
Zn,[BPO,]). Rulmond and Tarter" (1988) studied the borophosphates M"[BPO,] synthesized by Bauer [',21 and assumed
that they crystallized in the stillwellite type[61(Ce[BSiO,]). The
neutral borophosphate BP0,[7
is of particular interest because of its isostructural relationship to the polymorphic modifications of SiO, .[I9]
Ca[BPO,] and Sr[BPO,] were obtained as single-phase microcrystalline powders, as outlined in Equations (a) and (b) respectively, in platinum crucibles at 900 "C, following the procedures
described by Bauer.'
2 H,O
+ H3B0,
+ H,BO, + (NH,),HPO,
+ 4 H,O
+ 3 H,O + 2 NH, + COL
Well-ground mixtures were used in the mole equivalents given
in Equations (a) and (b). Ca[BPO,] was synthesized by heating
the reaction mixture at a rate of 2 Kmin-' to 900°C and then
cooling to room temperature at the same rate. For the synthesis
of Sr[BPO,J the reaction mixture was heated at 2 Kmin-' to
600 "C, cooled to room temperature, and ground up again; the
reaction was completed by heating at 900 "C for 24 h. According
to DTA experiments the borophosphates melt incongruently at
1025 "C (Ca[BPO,]) and 1092°C (Sr[BPO,]).
Single-phase samples of the compound Ba[BPO,] could not be
synthesized under the same conditions. Experiments employing
an excess of (NH,),HPO, as a flux medium or a solvent provided single crystals of the new compound Ba,[BP,O,,]: mixtures
of BaCO,, H,BO,, and (NH,),HPO, in the mole ratio 2:1:3
were heated to 1300 "C over 4 h in a platinum crucible and were
kept at this temperature for 8 h. After a further 24 h at 1000 "C
the mixtures were cooled to room temperature over 4 h. A glassy
matrix was formed in which were embedded elongated orthorhombic prisms of Ba,[BP,O,,]; Ba,[P,O,] was detected as an
additional crystalline side product by X-ray powder diffraction
studies. The crystal structures of the isostructural phases
Ca[BPO,] and Sr[BPO,] were refined with X-ray powder datarto1
(see also Fig. 1); the crystal structure of Ba,[BP,O,,] was determined by single-crystal methods.[' ' ]
[*] Prof. Dr. R. Kniep, Dr. G. Gozel."] Priv.-Doz. D r B. Eisenmann.
Dr. C. Rohr, Dip1.-Ing. M. Asbrand, Prof. Dr. M. Kizilyalli It'
Eduard-Zintl-Institut der Technischen Hochschule
Hochschulstrasse 10, D-64289 Darmstadt (FRG)
Telefax: Int. code (6151)16-4073
[ '1
Angeii. Chrm. Int. E d Engl. 1994. 33. KO.7
This work was supported by the Pinguin-Stiftung (Dusseldorf). G. G.
thanks TUBITAK for a grant for a research stay at the TH Darmstadt.
Permanent address: Middle East Technical University
Department of Chemistry, TR-06531 Ankara (Turkey)
VCH V~rlagsgesrllschafrmbH, 0 - 6 9 4 5 1 Wc~rnbeim,1994
0570-0#33/94/0707-0749 $ 10.00+ .25,'0
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cu2nsen, clusters, bridge, calculations, sulfur, initio, ph3, selenium, coppel, new
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