The chemical behavior of two bis(3,5-dialkylpyrazol-1-yl)acetic acids and their coordination properties towards ZnCl 2 and FeCl 2 was studied. Reaction of bis(3,5-dimethylpyrazol-1-yl)acetic acid (bdmpza) (1) with ZnCl 2 and FeCl 2 gave the 2:1 complexes [(bdmpza) 2 Zn] (4) and [(bdmpza) 2 Fe] (5). Crystal structures of both complexes were obtained. In contrast, the sterically more hindered ligand bis(3,5-di-tert-butylpyrazol-1-yl)acetic acid (bdtbpza) (3) coordinates only once to zinc(II) resulting in the complex [(bdtbpza)ZnCl] (6). This provides a structural model complex for the active sites of zinc enzymes such as thermolysin or carboxypeptidase.[a]

The molecular structure of the previously reported species "[Fe(bdtbpza)Cl]" has been revealed by X-ray structure determination to be a ferrous dimer [Fe(bdtbpza)Cl](2) (2c) [bdtbpza = bis(3,5-di-tert-butylpyrazol-1-yl)acetate]. The syntheses of ferrous 2:1 complexes [Fe(bpza)(2)] (3a) and [Fe(bdtbpza)(2)] (3c) as well as ferric 1:1 complexes [NEt(4)][Fe(bpza)Cl(3)] (4a) and [NEt(4)][Fe(bdmpza)Cl(3)] (4b) [bpza = bis(pyrazol-1-yl)acetate, bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetate] are reported. Complexes 3a, previously reported [Fe(bdmpza)(2)] (3b), and 3c are high-spin. No spin crossover to the low-spin state was observed in the temperature range of 5-350 K. 4a and 4b are synthesized in one step and in high yield from [NEt(4)](2)[Cl(3)FeOFeCl(3)]. 4a and 4b are iron(III) high-spin complexes. Crystallographic information: 2c (C(24)H(39)ClFeN(4)O(2).CH(2)Cl(2).CH(3)CN) is triclinic, P1, a = 12.171(16) A, b = 12.851(14) A, c = 13.390(13) A, alpha = 98.61(9) degrees, beta = 113.51(11) degrees, gamma = 108.10(5) degrees, Z = 2; 3a (C(8)H(7)Fe(0.5)N(4)O(2)) is monoclinic, P2(1)/n, a = 7.4784(19) A, b = 7.604(3) A, c = 16.196(4) A, beta = 95.397(9) degrees, Z = 4; 3c (C(24)H(39)Fe(0.5)N(4)O(2)) is monoclinic, P2(1)/n, a = 9.939(6) A, b = 18.161(10) A, c = 13.722(8) A, beta = 97.67(7) degrees, Z = 4; 4b (C(20)H(35)Cl(3)FeN(5)O(2)) is monoclinic, C2/c, a = 30.45(6) A, b = 12.33(2) A, c = 16.17(3) A, beta = 118.47(5) degrees, Z = 8.

The synthesis of the new chiral ligand (3,5-di-tertbutylpyrazol-1-yl)(3Ј,5Ј-dimethylpyrazol-1-yl)acetic acid (bpaH tBu2,Me2 ) (4) has been achieved. Two different synthetic routes to its precursor 3,5-di-tert-butyl-1-[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-1H-pyrazole (bpm tBu2,Me2 ) (3) are reported. Deprotonation at the methylene group, followed by reaction with carbon dioxide, yielded a racemic mixture of 4. The chemical behaviour of bis(3,5-di-tert-butylpyrazol-1-yl)acetic acid (bdtbpzaH) (2) and the new chiral N,N,O scorpionate ligand 4 involving their coordination to zinc ions was studied. [Zn(bpa tBu2,Me2 )Cl] (5) was formed from a mixture of ZnCl 2 , 4 and base. Reaction of bis(3,5-di-tert-butylpyrazol-1-yl)acetic acid (bdtbpzaH) (2) with Zn(CH 3 ) 2 or Zn(CH 2 CH 3 ) 2 gave the alkylzinc complexes [Zn(bdtbpza)(CH 3 )] (6) and

The chemical behavior of two bis(3,5‐dialkylpyrazol‐1‐yl)acetic acids and their coordination properties towards ZnCl2 and FeCl2 was studied. Reaction of bis(3,5‐dimethylpyrazol‐1‐yl)acetic acid (bdmpza) (1) with ZnCl2 and FeCl2 gave the 2:1 complexes [(bdmpza)2Zn] (4) and [(bdmpza)2Fe] (5). Crystal structures of both complexes were obtained. In contrast, the sterically more hindered ligand bis(3,5‐di‐tert‐butylpyrazol‐1‐yl)acetic acid (bdtbpza) (3) coordinates only once to zinc(II) resulting in the complex [(bdtbpza)ZnCl] (6). This provides a structural model complex for the active sites of zinc enzymes such as thermolysin or carboxypeptidase. The good agreement with these active sites was demonstrated by a crystal structure of 6. To the best of our knowledge, this is the first report of zinc complexes with a monoanionic N,N,O‐tridentate ligand using a carboxylate O‐donor. Substitution of benzylthiolate for the chloride ligand 6 yielded [(bdtbpza)Zn(SCH2Ph)] (8) indicating that the bulky tert‐butyl groups do not prevent access to the zinc atom. Reaction of bdtbpza (3) with FeCl2 led to [(bdtbpza)FeCl] (7), which might serve as a structural model complex for the active sites of mononuclear non‐heme iron oxidases and oxygenases such as IPNS, DAOCS or CAS.

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