What's the point?

There's one question that I am always asked when I tell people my research area. It's asked in a variety of ways but they all mean the same thing: "What use is that?" And then I have to tell my inquisitor that trying to coordinate metals to group 15 Zintl clusters has absolutely no direct applications for the near future and I'm only doing it for interest's sake.

This question annoys me because the implication is always there that if research doesn't appear to have any specific applications, then that research must be pointless. And I just cannot understand this point of view. "Blue skies research" is vital if we are to gain a full understanding of the world. Plus just because we can't see any uses for a particular research area now doesn't mean there won't be any in the future. For example, lasers were described as "a solution looking for a problem" back in 1960 but now have a huge variety of applications: microscopy, surgery and printing to name but a few. Who's to say that in thirty years from now there won't be a problem that can only be solved with group 15 Zintl ions? And then wouldn't it look ridiculous if all research into Zintls stopped in 2011 because there were no uses for them at the time? In my opinion, no research can ever be pointless.

Pointless?

All my own work part 2: Indium (again) and Thallium

This post is about what I spent the first year of my DPhil doing. Between Bryan Eichhorn's work with group 6 and group 10 transition metal compounds and the work I did for my Part II project with Cu, Zn, Cd and In compounds, the reactivity of group 15 Zintl ions towards organometallic reagents has been well studied. However, interestingly, there didn't seem to be any research on the reactivity of E₇^3– towards simple metal halide salts. This seemed strange for a couple of reasons. Firstly, practically every metal has at least one comercially available halide, whereas many organometallic compounds cannot be purchased and have to be synthesised in the lab. Secondly, the syntheses of many organometallics start from the metal halide, therefore surely it would be logical to try reacting E₇^3– with the metal halide first, before going to the trouble of synthesising the organometallic compound? I therefore started my DPhil looking at the reactivity of group 15 Zintl ions towards metal halide salts, namely the group 13 chlorides InCl₃ and TlCl.

Both K₃P₇ and K₃As₇ react with InCl₃ in ethylenediamine and in the presence of 2,2,2-crypt to form [K(2,2,2-crypt)]₃[In(E₇)₂] (E = P, As). These compounds contain the In-bridged cluster anions [In(P₇)₂]^3– and [In(As₇)₂]^3–. The crystal structure of [In(P₇)₂]^3– shows two P₇^3– cages bridged by an In atom. Each P₇^3– is bonded to the In in an η²-fashion, resulting in a distorted tetrahedral coordination geometry. [In(P₇)₂]^3– is isoelectronic with the [Zn(P₇)₂]^4– and [Cd(P₇)₂]^4– species I discussed in my previous post.

[In(P₇)₂]^3–.

K₃P₇ and K₃As₇ also react with TlCl in ethylenediamine and in the presence of 2,2,2-crypt or 18-crown-6 to form [K(2,2,2-crypt)]₂[TlP₇] and [K(18-crown-6)]₂[TlAs₇], which contain the Tl-functionalised cluster anions [TlP₇]^2– and [TlAs₇]^2–. The crystal structures of both species show an E₇^3– cluster bonded to a Tl atom in an η²-fashion.

[TlP₇]^2– and [TlAs₇]^2–.

So that's another small taster of the research I've been doing. Why not read the paper if you want to know more? I will hopefully be writing about more of my own research in the future, depending on how the experiments I'm doing at the moment pan out. Watch this space!

All my own work part 1: Copper, Zinc, Cadmium and Indium

I've been promising a post about my own research for the past couple of weeks, so here you are! This post is about the research I did for my Part II project as a fourth year undergraduate back in 2008/09. (Fourth year Chemistry undergrads at Oxford spend the year working in one of the research labs. Which is awesome and should be implemented everywhere.) Anyway if you read my previous rather epic post you'll know that the reactivity of group 6 (Cr, Mo, W) and group 10 (Ni, Pd, Pt) transition metal compounds towards group 15 Zintl ions has been very well studied by Bryan Eichhorn, however very few other transition or post-transition metal compounds have been investigated. So I set out to fill in some of these gaps, and I started by looking beyond group 10 to groups 11 (Cu), 12 (Zn, Cd) and 13 (In). And this is what I found...

Both K₃P₇ and K₃As₇ react with Cu₅(mes)₅ in ethylenediamine and in the presence of 2,2,2-crypt to form [K(2,2,2-crypt)]₄[Cu₂(E₇)₂] (E = P, As). These compounds contain the novel cluster anions [Cu₂(P₇)₂]^4– and [Cu₂(As₇)₂]^4–. The crystal structure of [Cu₂(As₇)₂]^4–shows two As₇^3– cages bridged by a Cu₂²⁺ dimetallic centre. Each Cu atom is bound by one As₇^3– cage in an η⁴-fashion and by the other in an η¹-fashion. The two Cu atoms are also linked by a Cu-Cu bond, giving each Cu atom a coordination number of six.

[Cu₂(As₇)₂]^4–.

K₃P₇ and K₃As₇ also react with the group 12 organometallics MPh₂ (M = Zn, Cd) and 2,2,2-crypt to form [K(2,2,2-crypt)]₄[M(E₇)₂] (M = Zn: E = P, As; M = Cd: E = P). These contain the novel cluster anions [Zn(P₇)₂]^4–, [Zn(As₇)₂]^4– and [Cd(P₇)₂]^4–. The crystal structures of [Zn(P₇)₂]^4– and [Cd(P₇)₂]^4– both show two P₇^3– clusters bridged by a metal atom. Both P₇^3– cages are bound in an η²-fashion to the metal centre, resulting a a distorted tetrahedral coordiantion geometry. Each η²-P₇ acts as a four-electron donor, and the metal is formally in the +2 oxidation state with a d¹⁰ electron configuration. The metal centre therefore has eighteen valence electrons and is electronically saturated.

[Zn(P₇)₂]^4–.

[Cd(P₇)₂]^4–.

Moving on to group 13, K₃P₇ and K₃As₇ react with InPh₃, again in ethylenediamine and in the presence of 2,2,2-crypt to form the compounds [K(2,2,2-crypt)]₂[E₇InPh₂] (E = P, As), which contain the In-functionalised cluster anions [P₇InPh₂]^2– and [As₇InPh₂]^2–. The crystal structure of [P₇InPh₂]^2– shows a P₇^3– cage bound in an η²-fashion to a four-electron (fourteen if the d electrons are taken into account) [InPh₂]⁺ fragment, resulting in a distorted tetrahedral coordination geometry. The η²-P₇ acts as a four-electron donor, so that overall the In centre has eight (eighteen) valence electrons and is electronically saturated.

[P₇InPh₂]^2–.

So that's what I spent a year doing. Pretty cool, huh? If you want to know more about any of these compounds, why not read the paper? A second post about my research should be coming soon.

The story so far....

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 P₇^3–, although they seem to work for As₇^3–as well. Fritz studied these alkylation reactions further in the early 1990s, and synthesised several new examples of R₃P₇ (R = Et, ⁿBu, ⁱBu, SiH_3, P^tBu₂) (refs. 5 and 6). More recently, Milyukov found that P₇^3– 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. 

The two possible isomers of R₃E₇.

Bryan Eichhorn also studied the alkylation reactions of E₇^3– 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.

The three possible isomers of R₂E₇^–.

In 1996 Dieter Fenske showed that P₇^3– 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. 

P7[FeCp(CO)2]3.

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 E₇^3– towards compounds of the group 6 (Cr, Mo, W) and group 10 (Ni, Pd, Pt) metals has been particularly well studied. Eichhorn found that E₇^3– 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 E₇^3– 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. 

[P7Cr(CO)3]3–.

P₇^3– reacts with Ni(CO)₂(PPh₃)₂ to form [P₇Ni(CO)]^3–, which consists of a P₇^3– 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. 

[P7Ni(CO)]3–.

As₇^3– reacts with Pd(PCy₃)₂ to form [Pd₂(As₇)₂]^4– (ref. 14). This species can be thought of as two As₇^3– 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.

[Pd2(As7)2]4–.

Eichhorn also found that E₇^3– 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 E₇^3–cage acts as a four-electron donor to give sixteen-electron square-planar Pt(II) complexes. 

[P7PtH(PPh3)]2–.

Eichhorn has also studied reactions in which fragmentation of the E₇^3–cage occurs, resulting in the formation of larger heteroatomic cluster alloys. E₇^3–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–.

[NbSb8]3–.

Sb₇^3– exhibits a very different reactivity towards Ni(CO)₂(PPh₃)₂ to that of P₇^3– 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). 

[Sb₇Ni₃(CO)₃]^3–.

As₇^3– 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. 

[As@Ni₁₂@As₂₀]^3–.

Sb₇^3– 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. 

[Ni₅Sb₁₇]^4–.

Eichhorn also found that the reaction between As₇^3– 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. 

[Pd₇As₁₆]^4–.

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.

4. Fritz, 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.

7. Milyukov, V. A.; Kataev, A. V.; Hey-Hawkins, E.; Sinyashin, O. G., Russian Chemical Bulletin 2007, 56 (2), 298-303.

8. Charles, S.; Fettinger, J. C.; Eichhorn, B. W., Journal of the American Chemical Society 1995, 117 (19), 5303-5311.

9. Mattamana, S. P.; Promprai, K.; Fettinger, J. C.; Eichhorn, B. W., Inorganic Chemistry 1998, 37 (24), 6222-6228.

10. Ahlrichs, R.; Fenske, D.; Fromm, K.; Krautscheid, H.; Krautscheid, U.; Treutler, O., Chemistry-a European Journal 1996, 2 (2), 238-244.

11. Eichhorn, B. W.; Haushalter, R. C.; Huffman, J. C., Angewandte Chemie-International Edition in English 1989, 28 (8), 1032-1033.

12. Charles, S.; Eichhorn, B. W.; Rheingold, A. L.; Bott, S. G., Journal of the American Chemical Society 1994, 116 (18), 8077-8086.

13. Charles, S.; Fettinger, J. C.; Bott, S. G.; Eichhorn, B. W., Journal of the American Chemical Society 1996, 118 (19), 4713-4714.

14. Moses, M. J.; Fettinger, J.; Eichhorn, B. Journal of the American Chemical Society 2002, 124 (21), 5944-5945.

15. Kesanli, B.; Charles, S.; Lam, Y. F.; Bott, S. G.; Fettinger, J.; Eichhorn, B., Journal of the American Chemical Society 2000, 122 (45), 11101-11107.

16. Kesanli, B.; Fettinger, J.; Scott, B.; Eichhorn, B., Inorganic 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.

18. Kesanli, B.; Fettinger, J.; Eichhorn, B., Journal of the American Chemical Society 2003, 125 (24), 7367-7376.

19. Charles, S.; Eichhorn, B. W.; Bott, S. G., Journal of the American Chemical Society 1993, 115 (13), 5837-5838.

20. Moses, M. J.; Fettinger, J. C.; Eichhorn, B. W., Science 2003, 300 (5620), 778-780.

21. Moses, M. J.; Fettinger, J. C.; Eichhorn, B. W., Inorganic Chemistry 2007, 46 (4), 1036-1038.