I promised a review of the research on group 15 Zintl ions at the end of my last post, and here is that review! Most of the earliest research was carried out by Marianne Baudler and Hans Georg von Schnering in the late 1970s and early 1980s, and involved using alkyl halides to form the neutral trialkylated Zintl ions R3E7 (R = Me, iPr, SiMe3, SiPh3, GeMe3, SnMe3) (refs. 1-4). The vast majority of these reactions have been carried out with P73–, although they seem to work for As73–as well. Fritz studied these alkylation reactions further in the early 1990s, and synthesised several new examples of R3P7 (R = Et, nBu, iBu, SiH3, PtBu2) (refs. 5 and 6). More recently, Milyukov found that P73– also reacts with alkyl tosylates to form R3P7 (R = iPr, nBu, iBu, 3-C5H11, C6H13) (ref. 7). The R3P7 products can be one of two possible isomers: a symmetric isomer and an asymmetric isomer. 31P NMR studies have indicated that, for the majority of R3P7, a mixture of the two is formed. The ratios of asymmetric isomer to symmetric isomer depend on the steric bulk of the alkyl substituent, with bulkier substituents favouring the formation of the symmetric isomer, in which the three alkyl groups are furthest away from each other. In the case of (R3E)3P7 (E = Si, Ge, Sn; R = H, Me, Ph), only the symmetric isomer is formed, suggesting that the R3E group is too sterically demanding to form the asymmetric isomer.
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| The two possible isomers of R3E7. |
Bryan Eichhorn also studied the alkylation reactions of E73– and found that tetraalkylammonium salts, R4N+, can be used to form the dialkylated species R2E7– (E = P, As; R = Me, Et, nBu, PhCh2) (refs. 8 and 9). In this case there are three possible isomers that can be formed: two symmetric isomers (Structures I and II) and an asymmetric isomer (Structure III). 31P NMR studies have shown that one of the symmetric isomers is formed for all R2P7–. Using steric arguments it was concluded that Structure I is the structure adopted by these compounds.
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| The three possible isomers of R2E7–. |
In 1996 Dieter Fenske showed that P73– reacts with FeCp(CO)2Br to form the neutral compound P7[FeCp(CO)2]3 (ref. 10). This compound is analagous to the previously described compounds, and like (R3E)3P7, the 31P NMR data indicate that only the symmetric isomer is formed. This is because the FeCp(CO)2 groups are extremely bulky and favour the formation of the less sterically hindered symmetric isomer.
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Over the last twenty years, most of the research into the coordination chemistry of group 15 Zintl ions has been carried out by Bryan Eichhorn. The reactivity of E73– towards compounds of the group 6 (Cr, Mo, W) and group 10 (Ni, Pd, Pt) metals has been particularly well studied. Eichhorn found that E73– reacts with M(CO)3L to form the [E7M(CO3)]3– species (E = P, As, Sb; M = Cr, W: L = mesitylene; M = Mo: L = cycloheptatriene), in which the E73– cluster is bonded to the metal centre in an η4-fashion (refs. 11 and 12). [E7M(CO3)]3– are eighteen-electron species, in which the transition metal is in the zero oxidation state and the η4-E7 group is acting as a six-electron donor.
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P73– reacts with Ni(CO)2(PPh3)2 to form [P7Ni(CO)]3–, which consists of a P73– cluster bound to the Ni(CO) centre in an η4-fashion (ref. 13). As for the [E7M(CO3)]3–species, the η4-P7 cage acts as a six-electron donor and the Ni has eighteen valence electrons.
As73– reacts with Pd(PCy3)2 to form [Pd2(As7)2]4– (ref. 14). This species can be thought of as two As73– clusters bonded to a Pd2 centre in an η2,η2-fashion. The Pd atoms are in distorted square-planar coordination environments and are linked by an axial Pd-Pd bond.
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Eichhorn also found that E73– reacts with Pt(PPh3)2(C2H4) to form [E7PtH(PPh3)]2– (E = P, As), which can be considered as an η2-E7 group bound to a twelve-electron [PtH(PPh3)]+ fragment (refs. 13 and 15). The E73–cage acts as a four-electron donor to give sixteen-electron square-planar Pt(II) complexes.
Eichhorn has also studied reactions in which fragmentation of the E73–cage occurs, resulting in the formation of larger heteroatomic cluster alloys. E73–reacts with Nb(toluene)2 and with the group 6 complexes M(arene)2 (M = Cr: arene = naphthalene; M = Mo: arene = Me-naphthalene) to form the [ME8]n– species (M = Nb, Cr, Mo; E = As, Sb; n = 2, 3) (refs. 16-18). [NbAs8]3–, [NbSb8]3–, [MoAs8]2–, [MoSb8]3– and [CrAs8]3– have been identified as a result of this research. These species consist of crown-like E8 rings centred by a transition metal ion. [NbAs8]3–, [NbSb8]3– and [MoAs8]2– are formally sixteen-electron diamagnetic complexes, whereas [MoSb8]3– and [CrAs8]3– have seventeen valence electrons and are paramagnetic. The E8 ring possesses a formal 8– charge, which requires high oxidation state transition metals, namely M5+ for [NbAs8]3–, [NbSb8]3– , [MoSb8]3– and [CrAs8]3–, and M6+ for [MoAs8]2–.
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Sb73– exhibits a very different reactivity towards Ni(CO)2(PPh3)2 to that of P73– discussed earlier. The product of this reaction is the [Sb7Ni3(CO)3]3– species, which can be thought of as a ten-vertex cluster (ref. 19).
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[Sb7Ni3(CO)3]3–.
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As73– reacts with Ni(COD)2 to form [As@Ni12@As20]3– (ref. 20). This cluster anion consists of an icosahedral [Ni12(μ12-As)]3– fragment that resides at the centre of a dodecahedral fullerene-like As20 cage. [As@Ni12@As20]3– is remarkably symmetrical and has almost perfect icosahedral symmetry.
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[As@Ni12@As20]3–.
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Sb73– also reacts with Ni(COD)2, however in this case [Ni5Sb17]4– is formed (ref. 21). This cluster contains a [Ni(cyclo-Ni4Sb4)]
ring unit that resides inside a Sb13 bowl. The structure of [Ni5Sb17]4– is very similar to the previously mentioned [ME8]n– series of compounds, in which all atoms in the E8 ring are equidistant from the transition metal centre. The Ni atom in the centre of the [Ni(cyclo-Ni4Sb4)]
unit is also in the geometrical centre of the ring. This atom is bound to the four Sb atoms in the ring and also to one Sb atom of the Sb13 bowl, giving an overall square-pyramidal coordination geometry. Assuming that the central Ni atom is a Ni(II) centre and that the Sb atoms are acting as two-electron donors, this results in an electronically saturated eighteen-electron configuration.
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[Ni5Sb17]4–.
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Eichhorn also found that the reaction between As73– and Pd(PCy3)2 discussed previously gives low yields of a second cluster anion, [Pd7As16]4– (ref. 14). This species is similar to the [Ni5Sb17]4– cluster, in that [Pd7As16]4–
also contains a [M(cyclo-M4E4)]
cap. However, for [Pd7As16]4–, the corresponding bowl has a formula of Pd2As12, and the central Pd atom in the [Pd(cyclo-Pd4As4)]
subunit is shifted significantly from the middle of the ring, with shorter contacts to the As atoms than to the other Pd atoms.
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[Pd7As16]4–.
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And there you have it, a review of the research on group 15 Zintl ions! These reactions were all carried out before I got involved, so unfortunately none of the clusters featured were made by me (though I wish they were!). A post about my own research will be coming soon however...
References
1. Honle, W.; Von Schnering, H. G., Zeitschrift Fur Anorganische Und Allgemeine Chemie 1978, 440 (1), 171-182.
2. Von Schnering,
H. G.; Fenske, D.; Honle, W.; Binnewies, M.; Peters, K., Angewandte Chemie-International Edition in
English 1979, 18 (9), 679-679.
3. Baudler,
M.; Faber, W.; Hahn, J., Zeitschrift Fur
Anorganische Und Allgemeine Chemie 1980,
469 (10), 15-21.
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G.; Hoppe, K. D.; Honle, W.; Weber, D.; Mujica, C.; Manriquez, V.;
Von Schnering, H. G., Journal of
Organometallic Chemistry 1983, 249 (1), 63-80.
5. Fritz, G.; Schneider, H. W. Zeitschrift Fur Anorganische Und Allgemeine Chemie 1990, 584 (1), 12-20.
6. Fritz, G.; Layher, E.; Goesmann, H.; Hanke, D.; Persau, C. Zeitschrift Fur Anorganische Und Allgemeine Chemie 1991, 594 (1), 36-46.
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V. A.; Kataev, A. V.; Hey-Hawkins, E.; Sinyashin, O. G., Russian Chemical Bulletin 2007,
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R.; Fenske, D.; Fromm, K.; Krautscheid, H.; Krautscheid, U.; Treutler, O., Chemistry-a European Journal 1996, 2 (2), 238-244.
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B. W.; Haushalter, R. C.; Huffman, J. C., Angewandte
Chemie-International Edition in English 1989, 28 (8), 1032-1033.
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Chemistry 2004, 43 (13), 3840-3846.
17. Eichhorn,
B. W.; Mattamana, S. P.; Gardner, D. R.; Fettinger, J. C., Journal of the American Chemical Society 1998, 120 (37),
9708-9709.
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21. Moses, M. J.;
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