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Hf7P4 A New Binary Phosphide Synthesized by a Surprising Route.

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Hf,P4 : A New Binary Phosphide Synthesized
by a Surprising Route**
Holger Kleinke and Hugo F. Franzen*
Dedicated to Professor Hans Georg von Schnering
on the occasion of his 65th birthday
Despite numerous investigations of binary, ternary, and even
quaternary pnictides and chalcogenides of the early transition
metals, new binary compounds in these systems can still be
found. Most recently, for example, Zr,P,['] Zr, +xTe,,[21
Hf,Se, J3] Hf,Te, J4] Nb,Te, Jzl Ta,S, J5] Ta,SeJb1 ?a,Te, J7]
and Ta,Te,@] have been synthesized. Thereby, the differences
between the metal-rich chemistry of Hf and Zr as well as between N b and Ta became more and more apparent: none of these
compounds have a counterpart among the corresponding homologous metal atom of the other period, and the stoichiometric equivalent phosphides Zr,P['] and Hf,P['] (or Nb,P1"] and
Ta2P[111)crystallize in different structure types; the same is also
true for Nb,Te, and Ta,Te,, as well as for ZrzS[lZ1and Hf2S.['31
As for some ternary Nb/Ta sulfides['41 and tellurides,[151the 5d
metal atoms prefer the positions with the higher metal-metal
bond order. Similarly, the binary chalcogenides of the 5d metal
atoms form structures with more metal-metal contacts (compare for example Zr,S with Hf,S, and Nb,Te, with Ta,Te,).
It was therefore surprising to find that Hf7P4[' is isostructural to Zr7P4.[191Hf7P4was prepared by arc-melting of Hf, Fe,
and HfP in the ratio of 5:2:4 by using techniques previously
described.[201The powder diagram obtained from the ground
bulk sample contained only the reflections of Hf,P4, HfFe,, and
Hf4FeP, which is consistent with a reaction according to Equation (a).
5Hf + 2 F e + 4 H P
6/7Hf,P, +5/7HfFe, +4/7H,FeP
Single crystals of Hf,P, were obtained after heating the bulk
sample in an induction furnace at a temperature of 1400 "C for
six hours. This shows that Hf,P4 is thermodynamically stable;
its synthesis is only kinetically impeded. Our attempts to prepare Hf7P4 without the addition of Fe led only to mixtures of
Hf,P['O1 and Hf,P, .fZ1]Thus, since the EDS (Electron Dispersive Spectroscopy) investigation of the measured single crystal
did not show a measurable amount of Fe, we conclude that there
is no Fe incorporation into the structure of Hf7P4, which is in
agreement with the uniformity of the equivalent temperature
factors of the structure model of Hf,P4. Since the largest void in
the structure is surrounded by four Hf and two P atoms at
(partial) occupation of this void by
distances of about 2.2
iron can be excluded for steric reasons: a Hf-Fe distance of
2.2 A would correspond to a bond order of 4.1 according to
Pauling's Equation (b), using r,,f =1.452 8, and rFe = 1.170 8,
significant amounts of impurities in Hf7P4. 2) After the arcmelting had been carried out under an argon atmosphere, the
annealing was performed under residual pressure of
thus significant incorporation of heteroatoms is impossible.
In addition, if one takes into consideration a stabilization of
Hf7P4 by oxygen, whereever it might come from, an analysis of
the voids in the structure does not show any probable positions
for oxygen: either the H f - 0 distances would be too short
(< 1.76 8, in the Hf, octahedra, compared to H f - 0 bonds of
2.05 8, in the structure of monoclinic Hf02)[231o r P - 0 distances less than 2.4 8, would occur. The latter is very unlikely
when P and 0 are considered as anions with filled shells. In
any case, no significant electron density was found in the considered voids.
However, the presence of Fe, which acts as a flux, seems to be
essential for the formation of Hf,P4. This is presumably the
reason why, in spite of studies of the formation of Hf-rich binary"41 and ternary p h o ~ p h i d e s , ' ~the
~ ] existence of Hf,P4 has
not been demonstrated up to now. Although the technique of
adding a flux material to prepare suitable crystals of metal-rich
compounds has been applied successfully in the synthesis of
Hf,Te, (using KC1)[41and Zr,S (using Cr),Iz6Imost of the previously mentioned pnictides and chalcogenides were prepared
by reducing a more metal-poor compound with the corresponding metal, o r from the elements directly, which led to very small
crystals in most cases. To our knowledge, the synthesis of Hf7P4
is the first example in which the use of a flux material has
enabled the synthesis of a metal-rich phosphide that has eluded
synthesis without using flux.
Hf7P4 crystallizes in the Nb7P4 structure type, consisting of
fragments of the body-centered cubic packing of Hf. The four
crystallographically different P atoms are located in singly, twoand three-capped trigonal Hf risms, and the Hf-P bonds range
between 2.57(2) and 3.24(2) . Alternatively, this structure can
be described in terms of condensed fragments of the M,X, cluster type.[271Two different variations of the condensed M,X,
cluster occur in the structure of Hf7P4,both kinds are condensed
through common vertex atoms to form infinite chains running
parallel to [OIO]: a Hf, fragment of the Hf,P, cluster, in which
one Hf atom in the basal plane of the octahedron is missing, is
situated between two crystallographically independent Hf,P,
octahedra, connected through equatorial Hf atoms to form
zigzag chains parallel to [I011. These chains are highlighted by
bold lines in Figure 1. Furthermore, these chains are intercon-
d(n) = d(t) -0.6 Ig
(n = bond order).[221Besides the results of the EDS investigation, a statistical presence of other heteroatoms is most unlikely
for the following reasons: 1) The high yield argues against
Prof. Dr. H. F. Franzen, Dr. H.Kleinke
Ames Laboratory DOE. Iowa State University
Ames, [ A 50011 (USA)
Fax: Int. code t(515) 294 5718
e-mail. franzen(rr
H. K. thanks the Deutsche Forschungsgemeinschaft for financial support of
this work. The Ames Laboratory IS operated for the US Department of Energy
by Iowa State University under Contract No. D-7405-Eng-82. This research
was also supported by the Office of the Basic Energy Sciences. Materials SCIence Division. Department of Energy.
0 VCH,s~esell.s~hrrfr
mhH, 0-69451 Wcmheim. 1996
Fig. 1. Projection ofthe structure of Hf,P, along [OlOj. the Hf, and H f , cluster units
are highlighted. Small. black circles: P; large, whlte circles: Hf.
o5?0-0XS3i96iS51?-/934 $ 15.00+ .2S/O
AnRea. Chem. Int. Ed. Engl. 1996, 35,
No. 17
nected by common P atoms and Hf-Hf bonds between the
cluster units. Thus. all Hf atoms are either part of the Hf, or of
the Hf, units.
The shortest Hf-Hf bonds occur between the apical and
basal atoms of the condensed cluster units (ranging from
3.025(3) to 3.194(2)
whereas the distances within the equatorial planes are significantly longer (ranging between 3.437(4)
and 3.684(4) A). As can be seen by the length of the b axis, the
octahedra are tetragonally compressed, and the Hf-Hf distances between the vertices are 3.5209(9)
Therefore a consideration of these units as centered cubes is appropriate, but this
model should not be taken too far because the Hf-Hf distances
between the cubic units (varying from 3.223(4) to 3.413(4) A)
are even shorter than those within the cubes.
Most unusually, in comparison with other metal-rich systems
of neighboring 4d and 5d elements, this metal-rich compound is
isostructural to its 4d homologue Zr,P4,[191although, based
upon prior comparison of Group 4 and 5 homologues,[281Hf
should prefer a structure with more metal-metal bonding than
in the Zr compound.
The observed contraction of the unit cell volume of almost
5 O/O from Zr7P4to Hf7P4cannot be explained only by the different radii of Zr and Hf (1.452 vs. 1.442A).r221Although the
differences between the three different crystallographic axes
vary between 1 and 3 O h (b axis), no significant trend is observed
among the different interatomic interactions. The greatest deviations occur in the Hf-Hf separations parallel to [OIO], which
correspond to the length of b and therefore have only weak
bonding character. All other types of bonds are about 1.5%
smaller in case of Hf7P4.
To investigate the consequences of the cell contraction, we
calculated the Pauling bond orders with Equation (b)[z21for
Zr,P4 and Hf7P4,and the Mulliken overlap populations (MOP)
for Hf7P4and for theoretical Hf7P4with the cell dimensions and
atomic positions of Zr7P4 by using the extended Hiickel
method.[zg1The parameters for Hf and P were taken from the
-12 0 -
-14 0 --.
-16 0
I 1
Fig. 2. COOP curve of Hf,P,; solid line: Hf-HT; dashed line- Hf - P interactions.
The left half of the diagram covers the antibonding, the right half the bonding
The calculated COOP curve (crystal orbital overlap population) consists of two well-separated parts (excluding the phosphorus 3s orbitals below the energy window): at lower energy,
between - 14.5 and - 12 eV, the 3p states of P dominate, leading, by mixing with Hf 6s. 6p and 5d states, to strong bonding
Hf-P interactions, whereas in the region of the Fermi level EF
mainly the Hf d bonding states occur. In contrast to more metalrich compounds[311not all metal-metal bonding states are filled
(Fig. 2).
The calculated COOPS of Zr,P4 and Nb7P4 (parameters
taken from literature)1321are very similar. Because of the
higher electronegativity of N b its d block is shifted to lower
energies and thus overlaps with the 3p block of P. The higher
number of valence electrons leads not only to completely
filled Nb-Nb bonding states, but also to the occupancy of
some antibonding states (Fig. 3). Altogether, the Nb-Nb and
Nb-P overlap populations are significantly larger than those of
After the cell contraction from Zr,P4 to Hf7P4,all atomic sites
show the same trend, that is, the metal-metal overlap populations increase by 2.8% from theoretical Hf,P4 in the Zr,P4
structure (average total MOP,,-,, = 0.996) compared to
those calculated for the structure model of Hf,P4 (average
MOP,, - HI = 1.024), and an increase is also found for the metal nonmetal overlap population (0.948 vs. 0.964, corresponding to
an increase of 1.7 Y O )Accordingly,
the averaged Pauling metal metal bond orders changed from 2.56 (Zr7P4) to 2.77 (Hf7P4)
and the metal -P bond orders from 2.44 to 2.70, indicating that
18 0
Fig. 3. COOP curve of Nb,P,: solid line: Nb-Nb; dashed line: Nb -P interactions.
The left half of the diagram covers the antibonding. the right half the bonding
the cell contraction is not caused by the smaller radius of Hf
The higher metal-metal bond order in the structure of Hf7P4
can be explained by the greater extension of the 5d orbitals as
well as by the electron concentration rules of Brewer and Engel.[331According to Brewer and Engel, the s and p conduction
electrons determine the structure, and Zr and Hf tend in bodycentered cubic structures to the configurations of d2.7spo and
d2.5spo.5,respectively; thus, the higher ratio of s and p electrons
of Hf enables Hf to form stronger metal-metal bonds.
On the other hand, the increase of metal -metal bonding leads
to a lower energy of bonding orbitals centered on Hf, which in
turn leads to a higher “back-bonding” covalent character of the
Hf-P bonds, corresponding to the higher overlap population.
In other words, Hf is more electronegative than Zr and thus
forms more covalent M - P interactions.
Relativistic effects in general lead to a radial contraction of
the s and p orbitals and to an expansion of the d
is particularly true in the case of Hf because of the increase of
relativistic effects with Z2.[351The result is a volume contraction
of the Hf phosphide because of its smaller, structure-determining s a n d p orbitals. However, since the radii used for calculating
the bond orders are based on experimental results, and the experiments are necessarily relativistic, relativistic effects are included by comparisons based on radii. Therefore, the significant
higher bond orders of Hf,P4 indicate that relativistic effects
alone are not responsible for the observed cell contraction.
All viewpoints complement one another and lead to the conclusion that compared with Zr, Hf will be more strongly bonded
to itself in metal-rich compounds and more covalently bonded
to the nonmetal. In general, these conclusions should also be
true for the structural differences of metal-rich compounds of
Nb and Ta. In summary, the general trend in the different bonding behavior of Zr and Hf, observed in many examples with very
different crystal structures, can also be studied based on
isostructural compounds like Zr7P4 and Hf7P4.
Received March 14, 1996 [Z8927IE]
German version: Angeu Chem 1996, 108. 2062-2064
Keywords: hafnium compounds orbital populations
phorus compounds solid-state structures
Photoinduced Energy and Electron Transfer in
Supramolecular Porphyrin Assemblies**
Christopher A. Hunter* and Robert K. Hyde
[I] P:J. Ahlzen. S. Rundqvist, 2. Kristullogr. 1989, 189, 117-124.
[2] H. Kleinke, W. Tremel. 25. Huuptsersummlung der Gesellschuft Deutscher
Chemiker (Munster) 1995, Abstr Pap. p. 240.
[3] I . M. Schewe-Miller, V. G. Young, Jr., J. A//oys Compd. 1994. 216. 113-115.
[4] R. L. Abdon, T. Hughbdnks. Angew. Chein. 1994. 106, 2414-2416; Angeir.
Chem. Int. Ed. Eng/. 1994, 33, 2328-2330.
[ 5 ] H. Wada. M. Onada, Marer. Res. Bull. 1989, 24, 191-196; S:J. Kim. K. S.
Nanjundaswamy, T. Hughbanks, Inorg. Chem. 1991, 30, 159-164.
161 B. Harbrecht. Angeu. Chem. 1989, 101, 1696-1698; Angew. Chem. Int. Ed.
Engl. 1989.28, 1660-1662.
[7] M. Conrad, B. Harbrecht. J. Alloys Compd. 1992, 187, 181-192.
[8] M. Conrad, 8 . Harhrecht, lVth Eur. Con/. Solid Stare Chem. (Dresden) 1992.
Abstr. Pap. p. 324.
[9] T. Lundstrom, N.-0. Ersson. Acta Chem. Scund. 1968, 22. 1801 -1808.
[lo] Y. B. Kuzma. S. V. Orishchin, Y F. Lomnitskaya, T. Glovjak. Dopov. Akud.
Nauk Ukr. R S R Ser. B: Geol. Khim. Biol. Nuukr 1988.2.47-49.
1111 A. Nylund. Acru Chem. Scund. 1966.20, 2393-2401.
1121 B. R. Conard, H. F. Franzen, High Temp. Scr. 1971, 3,49-55.
[13] H. F. Franzen, J. Graham, 2. Kristullogr. 1966. 123, 133-138.
[14] X. Yao, H. F. Franzen,J. SolidState Chem 1990.86.88-93; J. A m . Chem. Soc.
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[15] H. Kleinke. W. Tremel, 2. Kristullogr. S ~ p p l 1992,
7, 139.
[16] Crystal structure analysis OF Hf,P,: lattice constants for the monoclinic cell
(space group C2/in) were obtained from Guinier powder patterns under vacuum (50 reflections. Cu,,): u =15.488(5), h = 3.526(1). c =14.553(5) A,
fl = 104.83(3)’. V =768.3(8) A’, and from refinements of 14 centered retlecMo,,): u = 15.654(6),
tions with the AFC6R diffractometer (RIGAKU, 23 ‘T,
b = 3.5209(9), c =14.596(7) A. fl =104.43(3)”, V =779(1) A’. Experimental
details of data collection: 2 = 4, p = 11.872gcm-’. { I = 940.89cm-I.
28,,, = 7 0 , min.lmax transmission: 0.61/1.31. Refinements and absorption
correction (Y scan, followed by DIFABS [17]) were carried out using the
TEXSAN program package [18]. Final residual values are R ( F ) = 0.058.
Rir = 0.056, GOF =1.386 with 669 independent observed reflections
( I z 341)) and 57 variables. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, D-76344
Eggenstein-Leopoldshafen (Germany), on quoting the depository number
[17] N. Walker, D. Stuart, Actu Crjsfullogr. Sect. A 1983, 39. 159-166.
[18] TEXSAN: Single Cr~sralStructure Analysb Sofiware, Version 5.0, Molecular
Structure Corporation, The Woodlands, TX, USA, 1989.
[I91 P:J. Ah1zt.n. S. Rundqvist. 2. Kristullogr 1989. 189. 149-153.
[20] H. Kleinke, H. F. Franzen, 2. Anorg. Allg. Chem. 1996. in press.
[21] T. Lundstrom. Actu Chein. Scand. 1968, 22, 2191 -2199.
[22] L. Pduling, The Nurtrre of fhe Cheinicul Bond. 3rd ed., Cornell University Press,
Ithaca, NY. 1948.
1231 C. E. Curtis, L. M. Doney. J. R. Johnson. J. Am. Cerum. S~JC.
1954, 37, 458465.
1241 T. Lundstrom. P. Tansuriwongs. Acru Chem. Scund. 1968.22, 704-705.
(251 G. J Miller, J. Cheng, Inorg. Chem. 1995, 34. 2962-2968; J. Cheng, H. F.
Franzen. J. Solid Slate Chem. 1996. 121. 362-311.
[26] X. Yao. H. F. Franzen. J. Less-Common Met. 1988, 142, L27-L29.
[27] A. Simon. A n g w . Cliem.1981, 93, 23-44; Angel%,.Chem. hit. Ed. Engl. 1981,
20, 1-22.
[28] H. F. Franzen. M. Kockerling. Prog. Solid State Chem. 1995, 23, 265-289.
1291 R. Hoffmann, J. Clzem. P1t.v.s. 1963.39.1397- 1412; M.-H. Whdngbo, R. Hoffmann. J. A m . Chem. Soc. 1978,100.6093-6098; R. Hoffmann, Angew. Chem.
1987, 99. 871--906, Angew. Chc,m. Int. Ed. Engl. 1987.26, 846-878.
[30] E. Clementi, C. Roetti. A t . Dufu Nucl. Datu Tables 1974, 14. 177-478.
1311 T. Hughbanks. Prog. So/idState Chenr. 1989, 19, 329-372.
(321 R. H. Summerville, R. Hoffmdnn, J. Am. Clzem. SOC.1976, 98, 724-7255,
1331 L. Brewer in Allo-ving (Eds.: J. L. Walter, M. R. Jackson, C. T. Sims), ASM
International, Metal Park. OH, 1988, Chapter 1.
[34] P. Pykko, Chenr.Rev. 1988. 88. 563-594.
[35] P. Pykko, J. G. Snijders. E. J. Baerends, Chem. Phys. Lett. 1981, 83,432-436.
$> VCH Verlagsgescllsehqfi mbH. 0.69451 Weinheim. 1996
Nature uses complex arrays of chromophores to trap solar
energy and convert it into the chemical potential that drives the
chemistry of photosynthetic organisms.“] These chromophores
are all noncovalently attached to a protein scaffold which holds
them at the right separation and orientation for fast energy and
unidirectional electron transfer, so that the solar energy conversion process proceeds with a high quantum yield. If we are to
mimic the functional properties of these systems, it is likely that
we will have to develop strategies for the construction of similarly large but structurally well-defined multichromophore arraysc2-41 Hydrogen bonding has been successfully employed in
the construction of photoactive supramolecular assemblies, but
these complexes are relatively unstable with association constants in the region of lo3 M - and this limits the concentration
range over which they can be studied as well as their ultimate
Complexes assembled through porphyrin coordination chemistry suffer from the same problem, and this has hampered detailed analysis of their photochemical
describe here a new approach that overcomes this problem: the
use of cooperative coordination interactions to generate very
stable self-assembled chromophore arrays.r51The porphyrin
units employed in these arrays are shown in Scheme 1.
The design of the two complexes shown in Figure 1 is based
on the structure of a self-assembling porphyrin “dimer” which
we described recently.[6, ’I Zn,-2 contains a naphthalenediimide
group that can act as an electron acceptor in photoinduced
electron transfer reactions with porphyrins.[’I Zn,-3 has the
same overall dimensions as Zn,-2 but has in place of the naphthalenediimide a terephthaloyl diamide, which has a much
higher reduction potential and therefore cannot act as an electron acceptor: for our purposes, it behaves as a n inert spacer.
H,-1 is a rigid bifunctional ligand of appropriate dimensions to
bridge the two zinc centers in the porphyrin “dimers” as shown
in Figure 1 .
Dr. C. A. Hunter. R. K. Hyde
Krebs Institute for Biomolecular Saence, Department of Chemistry
University of Sheffield
Shefield S37HF (UK)
[**I We thank the Lister Institute (C. A. H.), the University o f Shefield, and
Zeneca Specialities (R. K. H) for financial support.
0570-0833/96/3517-1936 $ 15.00i .25/0
Angeu. Cliem. lnt. Ed. Engl. 1996, 35. N o . 17
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