Polypyrazolylborates: Scorpionates - Journal of Chemical Education

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George B. Kauffman California State University Fresno, CA 93740

Polypyrazolylborates: Scorpionates Swiatoslaw Trofimenko Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716; [email protected]

In introductory chemistry courses the concept of the chemical bond is of major importance. Various types of chemical bonds such as ionic, covalent, and coordinate covalent are presented and explained, and shown in what systems and compounds they are found. It is pointed out that numerous chemical compounds are actually coordination complexes. Many of them, based on rather complicated ligands, are important in major life processes. For instance chlorophyll, the ultimate basis of all life on the planet, is a coordination compound of magnesium; vitamin B12 is a coordination compound of cobalt; hemoglobin in our blood is a coordination compound of iron; hemocyanin of invertebrate blood is a coordination compound of copper; and there are many other examples. Coordination compounds and complexing agents are used in everyday life as, for instance, in extraction, dyeing, leather tanning, electroplating, catalysis, water softening, and other applications. Although in introductory chemistry courses simple ligands such as water, ammonia, chloride, and other inorganic ions are discussed, there are more complex and unusual ligands that can motivate, interest, and excite beginning students. In organometallic and coordination chemistry the choice of a ligand is important, whether to effect catalytic activity for some chemical reaction, to model a bioinorganic enzyme system, or to achieve other goals. Among the most often used uninegative ligands the two outstanding examples are the cyclopentadienyl and the beta-diketonate anions, including their variants, such as substituted cyclopentadienyl ligands, or beta-diketonates where the oxygen atoms have been replaced by sulfur or by arylimido moieties. Polypyrazolylborate Anions In 1966 a versatile new class of ligands appeared, combining some features of the cyclopentadienide and betadiketonate ligands (1). About two thousand articles have been published on the chemistry of these new ligands, complexes of these ligands with 70 elements of the periodic table have been reported, a book covering the literature up to late 1990s (2) and numerous reviews (3–22) have been published covering this subject, and a special symposium was devoted to these ligands at the April 2003 ACS Meeting (23). Clearly these ligands should be an integral part of inorganic courses, both lecture and laboratory. And yet, this ligand class has received only scant attention in textbooks of inorganic chemistry. These new ligands are the polypyrazolylborate anions, of general structure [RnB(pz)4-n]−, where n can be 0, 1, or 2, pz is a pyrazol-1-yl group, and R can be H, an alkyl, or aryl group (Figure 1).

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R ″

4 5

Hn

B

3

N

N

1

2

R⵮

R ′ 4 −n

Rn

B

N

N

4 −n

Figure 1. General structure of polypyrazolylborate anions, [RnB(pz)4-n]−, where n can be 0, 1, or 2, pz is a pyrazol-1-yl group, and R can be H, an alkyl, or aryl group.

Synthesis These ligands can be synthesized by many different routes (2), but most easily by the neat reaction of pyrazole (or substituted pyrazole) with a borohydride anion in the absence of solvent. The reaction of unsubstituted pyrazole is shown as follows: −

− [BH4] + excess Hpz

[H2B(pz)2] or − [HB(pz)3] + nH2 or − [B(pz)4]

(1)

Temperature control steers this reaction to produce selectively only one of the above products. With asymmetric pyrazoles, such as 3-R-pyrazoles, this reaction proceeds with boron being bonded to the least hindered nitrogen atom. The only exceptions are indazole (benzopyrazole) and indazoles with substituents at positions other than 7, which bond through the more hindered nitrogen atom (24, 25). Naming When n is 1, and R is H, we have the parent tridentate ligand [HB(pz)3]−, which is analogous to the Cp ligand in being uninegative, providing six electrons, and occupying three coordination sites in its complexes. It has been given the abbreviation Tp, and its substituents are denoted by superscripts. The “default” position in this abbreviation system is the 3-position on the pyrazole ring (Figure 1), which is denoted by a superscript “R”, that is, TpR. Thus, the 3-phenyl analogue of Tp is written as TpPh (11).

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Bonding Since the systematic name poly(pyrazol-1-yl)borates is cumbersome and does not convey the mode of coordination of these ligands, the moniker “scorpionates” was coined (11), which provides an idea of how the ligand bonds. Almost always two pyrazolyl groups (claws) are coordinated to the metal ion (Figure 2), and the resulting six-membered ring is in a deep boat form that brings the third pyrazolyl (homo-

R⬘ R

B

M

N

N

N

N

scorpionates), or other (H, OR, SR, NR2) group (heteroscorpionates), like the tail of a scorpion, arching over and “stinging” the metal ion. The point here is that the “sting” is not mandatory, since the Tp ligand can sometimes be only bidentate (κ2), as in complexes of Rh(I) and Pd(II). Most of the work with scorpionates was done with the parent Tp ligand and with Tp* [= HB(3,5-Me2pz)3], both analogues of Cp. Compared to Cp, the Tpx ligand offers more choices of modifying its steric and electronic features by substitution on the pyrazolyl rings and on boron, having ten substitutable positions compared to just five for Cp. Moreover, while only R5Cp ligands retain the original symmetry of the Cp ligand, in the case of Tp many substitution types are possible with retention of the original C3v symmetry as, for instance, TpR, Tp4R, TpR2, TpR3, RTp,1 and many others. Even minor changes in substitution, such as replacement of a 3-H and 5-H by 3-Me and 5-Me, as in Tp* = TpMe2, can affect the chemistry; for instance, {Mo[HB(pz)3](CO)3]}− + [ArN2]+

Figure 2. Bonding in a polypyrazolylborate–metal complex.

TpMo(CO)2(N NAr) {Mo[HB(3,5-Me2pz)3](CO)3]}− + [ArN2]+ Tp*Mo(CO)2(η2-COAr)

N H

N

B

N

N N N

N M H

B

N N

N N N

N M

H

B

N N

N N

N M

H

B

N N

N

N N

M

N

Figure 3. Increasing hydrophobic environment around the metal ion owing to steric hindrance of the substituted polypyrazolylborate anion. Note that ⫺N – N⫺ is the third pyrazolyl ring.

(2)

(3)

Also note that the progressive increase in steric hindrance of Tp ligands can affect the depth of the hydrophobic pocket around the metal as shown in Figure 3. While the Tp and Tp* ligands readily yield octahedral complexes M[Tp]2 and M[Tp*]2 with first-row transition metals, the presence of a 3-t-butyl group in TptBu prevents formation of octahedral complexes and leads predominantly to tetrahedral species M[TptBu]X. The TptBu ligands have accounted for a number of striking new developments in coordination chemistry, such as stabilizing monomeric alkylmagnesium (26–28), alkylzinc (29, 30), and alkylberyllium species (31); novel oxygen complexes (32); and zinc-based enzyme models (33, 34). The Tl(I) salts of Tpx complexes are useful for the isolation and purification of the ligands, and they are soluble in organic solvents. Tl[Tpx] species are almost always monomeric in the crystal. A striking exception is the tetrameric {Tl[Tpcpr]}4, where cpr is cyclopropyl, the structure of which consists of a perfect tetrahedron of Tl atoms, each apex being capped by a Tpcpr ligand (Figure 4). The ease of synthesis of scorpionate ligands and of their complexes can be readily utilized in teaching inorganic synthesis. For instance, the synthesis of KTp* can be carried out in one laboratory session (35). Its conversion to the yellow [Tp*Mo(CO)3]− anion, followed by in situ treatment with butyl nitrite, yields the neutral orange complex TpMo(CO)2NO (36, 37). Hpz*(excess) + KBH4

(4) K[HB(pz*)3] (= KTp*) + 3H2 Figure 4. The structure of {Tl[Tpcpr]}4. Three of the apical Tpcpr ligands have been omitted, for the sake of clarity.

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KTp* + Mo(CO)6


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K[Tp*Mo(CO)3] + 3CO (5) www.JCE.DivCHED.org


K[Tp*Mo(CO)3] + BuONO

(6) TpMo(CO)2NO + CO + KOBu The denticity of a Tp ligand can be extended beyond κ3 by including 3-substituents with additional donor atoms. For instance, TpPy, which contains a 3-(2-pyridyl) substituent, is κ6 in its icosahedral complexes of U(III) and Sm(III) (38). An example of a hexadentate N3O3 ligand is Tp with a 3carboxypyrrolidido substituent, TpCONC4H8. While in crystals of its lanthanide complexes [TpCONC4H8]2(anion) one ligand is κ6 and the other κ4, with one arm detached, in solution there is a rapid exchange of the coordinated and free pyrazolyl arms so that all arms are magnetically equivalent (39). By contrast when the 3-substituent is carboxy-t-butylamido, the TpCONHtBu ligand is κ6 in the crystal as well as in solution of its in lanthanide complexes (Figure 5) (40). Applications of Borate Scorpionates

Figure 5. Structure of the cation {La[TpCONHtBu]2}+.

Some examples in which polypyrazolylborate ligands have been employed in the synthesis of complexes that had, inter alia, catalytic activity in various reactions, or served as models of biologically active complexes, are discussed briefly below.

Enzyme Modeling Complexes of diverse scorpionate ligands were used to mimic the activity of enzymes containing various metals, usually providing an environment similar to that of three coordinated histidine nitrogens. For instance vanadium complexes of general type Tp*VO(OAr)2 have been considered as possible models for the active site in bromoperoxidase (41). In the case of molybdenum, various Tp complexes were employed to model the mononuclear molybdenum cofactors of molybdopterin enzymes, such as xanthine oxidase–dehydrogenase, sulfite oxidase, nitrate reductase, and dimethyl sulfoxide reductase (42–48). Similar complexes were also employed in mimicking sulfur-containing tungsten enzymes, such as aldehyde oxidoreductase, formate dehydrogenase, and formylmethanofuran dehydrogenase (49–51). Mimicking the activity of manganese superoxide dismutase and of various binuclear manganese enzymes active in redox functions, was approached with TpiPr2Mn(OBz) and related binuclear complexes (52–54). In the area of iron-containing enzymes, the behavior of the oxo-bridged diiron enzyme hemerythrin was approximated with complexes such as [TpFe]2(µ-O)(µ-OOCR) and [TpFe] 2(µ-OH)(µ-OOCR) (55–58), while the complex TpiPr2Fe(OOCPh)(µ-O)(µ-OAc) was regarded as a synthetic model for the dioxygen binding site of nonheme iron proteins (59). Other related complexes were regarded as structural and functional models of catechol dehydrogenases (60) and of methanol monooxygenase (61). Complexes such as Tp*Ni(OEt-cysteinato) were studied as being of relevance to the nickel component of the active site in several hydrogenase enzymes, which participate in the bio-generation of hydrogen and methane, as well as in nitrogen fixation (62). Copper is present in a variety of enzymes, and the use of scorpionate–copper derivatives for modeling purposes has been reviewed (63). Specific complexes were used for modelwww.JCE.DivCHED.org

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ing the active site in phenylalanine hydroxylase (64), the copper–ethylene binding sites in plants (65), the copper center of galactose oxidase (66), in poplar plastocyanin (67), in blue copper proteins (68–71), in mixed-valence dicopper electron transfer sites of the enzymes nitrous oxide reductase or cytochrome c oxidase (72), and in oxyhaemocyanin and oxytyrosinase (73–75). Much work has also been done in zinc complexes related to carbonic anhydrase, and the complex TptBu,MeZn(OH) proved to be a good functional model for that enzyme (76, 77). The catalytic cycle of liver alcohol dehydrogenase was modeled by [TptBu,MeZn]L species (78), while other complexes mimicked the activity of alkaline phosphatase (79). The activity of cobalamine independent methionine synthase was approached through complexes such as Tp*ZnSR and TpPh,MeZnSR (80).

Catalysis Certain Tpx complexes were found to catalyze a variety of chemical reactions, including polymerization and oligomerization. For example, the complex Tp*W(⫽NPh) Br(⫽CHPh) and related complexes when combined with AlCl3 catalyzed the acyclic diene methathesis (ADMET) of 1,9-decadiene and also the ring-opening metathesis (ROMP) polymerization of cyclooctene (81–83). Similar activity was found in analogous molybdenum complexes (84). Terminal alkynes were dimerized by neutral Ru(II) complexes such as, for instance, TpRuCl(PPh3)2 (85), while TpRu(⫽C⫽CHPh) (Cl)(PPh3) achieved ROMP polymerization of norbornene (86). Phenylacetylene was homopolymerized by species such as TpR2Rh(COD) (87). The sterically hindered complex TptBuMg(OEt) was a precursor for rapid stereoselective ringopening polymerization of L,L-dilactide to yield isotactic poly(L,L-lactide) (88). Ethylene was polymerized by activated TpxNbMe2(PhC⬅CMe) (89), by Tp*V(⫽NAr)Cl2 (90), and by Tpx yttrium complexes that required no cocatalysts (91). Stereoregular copolymerization of ethylene and carbon monoxide was catalyzed by TpPhNi(PPh3)(o-Tol) (92).


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C–H Bond Activation

Conclusion

Activation of aliphatic and aromatic C⫺H bonds was by photolysis of the Tp*Rh(CO)2 complex, which proceeded with loss of CO and oxidative addition of ArH or RH (93– 96). Many other examples of C⫺H activation are listed in section 5.4 of ref 1. More recent examples employed TpxCu species to effect cyclohexane and benzene amination (97) and insertion of the CH2COOEt moiety into C⫺H bonds of hydrocarbons and ethers (98).

Carbene and Nitrene Transfer The complex Tp*Cu(ethylene) catalyzed the reaction of ethyl diazoacetate with olefins to form cyclopropanes and also catalyzed nitrene transfer from PhI⫽NTs to form aziridines (99–101). The cation [Tp*W(⫽CH2)(CO)(PhCCMe)]+ was also active in similar reactions (102). More recent work showed that the efficiency of carbene transfer by TpxCu complexes was related to the substituents on the Tp ligand (103, 104). Thus, the ligand TpMs (Ms is mesityl) was outstanding for the cyclopropanation of olefins (105), but in the carbene transfer to acetylenes TpCy (Cy is cyclohexyl) was the stellar performer (106).

Although polypyrazolylborates are archetypal examples of the scorpionate ligand system, related scorpionate ligands may be obtained by replacement of either the central boron or the pyrazole. A number of such examples have been covered earlier (35). Neutral analogues are best exemplified by geminal polypyrazolylalkanes that were introduced in 1970 (107). There are also non-pyrazolyl scorpionate ligands, containing a boron core. They are of the type [Ph2B(CH2Z)2]− and [PhB(CH2Z)3]−, where Z can be a variety of donor functionalities. Examples of Z = SR for both types (108, 109) of Z = PPh2 for [Ph2B(CH2Z)2 (110, 111), for [PhB(CH2Z)3]− (112, 113), and for Z = NMe2 (114), have been reported, as have those containing, in addition, a pyrazolyl substituent (115). The chelate ring size is another adjustable variable, leading to ligands such as [RB(CH2Z)2(ER)]−, where E = O, S, Se, or appropriate phosphino or amino functionalities. Related polypyrazolylborate examples, containing two pyrazolyl bridges and one dimethylamino (116), or arylmercapto (117), or alkoxy bridge (118), have been reported earlier. While chelates derived from the above ligands involve five- and six-membered rings between boron and the coordinated metal, other ligands lead to eight-membered rings. These ligands are represented by hydrotris-(mercaptoimidazolyl)borate (119, 120) and the related hydrotris(thioxotriazolyl)borate (121). The former contains twisted eight-membered rings in its complexes that are always sulfur bonded, while the latter is ambidentate and bonds either through sulfur (with bismuth or tin) or through nitrogen (sodium or manganese). Dihydrobis(thioxo-triazolinyl) borate, a still different ligand, bonds in tridentate fashion through two sulfurs plus an agostic hydrogen bond (122, 123).


Note 1. The superscripted R indicates substitution at the 3 position on the pyrazol group; superscripted #R indicates substitution at a position other than the third position; superscripted R# indicates multiple substitutions; and RTp indicates substitution of the H bonded directly to the boron atom.

Literature Cited

Other Scorpionate Ligands

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As we can see from the foregoing, scorpionate-type ligands and particularly the original polypyrazolylborates are easy to synthesize, have good stability, and are, in general, quite user-friendly. They also have well-defined spectroscopic tags in their NMR and IR spectra (sharp B⫺H stretch). Their thallium(I) salts are readily soluble in organic solvents, which permits their use in organic media, or in two-phase aquo–organic solvent mixtures. All these features should make them useful in the teaching of coordination chemistry, both in lectures and in the laboratory, providing relatively easy yet instructive examples of synthesizing coordination and organometallic complexes.

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