Hans Pritzkow

Cooperative Transformations of Small Molecules at a Dinuclear Nickel(II) Site

The synergetic action of two nickel(II) ions embedded in a tunable dinucleating ligand matrix (illustrated) allows manifold cooperative reactions within the bimetallic pocket, such as the hydration of nitriles, the hydrolysis of esters, and the degradation of urea to cyanate. Related mononuclear complexes have been prepared in order to further elucidate mechanistic aspects of these transformations.

Iron coordination chemistry with tetra-, penta- and hexadentate bispidine-type ligands

Described is the synthesis of tetra-, penta- and hexadentate bispidine ligands with two tertiary amine and two, three or four additional donors (pyridine, phenolate or alcoholate; bispidine/3,7-diazabicyclo[3.3.1]nonanone, coordinating substituents at positions 2,4; 2,4,7; 2,3,4; 2,3,4,7) and of their hexa-coordinate iron(II) complexes. Crystal structural analyses reveal that all complexes are six-coordinate, with one or two co-ligands, and all structures with the tetradentate bispidine ligand are asymmetrical with respect to the two tertiary amine donors, with short Fe� /N 1 and long Fe� /N 2 bonds (N 1 : position 3, N 2 : position 7). This is the same structural type as found for the Jahn/Teller labile copper(II) compounds, the manganese(II) and chromium(III) complexes but different from copper(I), zinc(II) and some cobalt(II) complexes with M � /N 1 ]/M � /N 2 . Additional donors at N 2 modify the structures, but do not lead to a change to the other structural type; additional donors at N 1 lead to structures with M� /N 1 � /M� /N 2 . Solution studies (NMR, UV/Vis, electrochemistry, magnetism) indicate that the co-ligands may be substituted by solvent, with the donors trans to N 2 being more labile than those trans to N 1 , but the over-all structural properties in solution are similar to those in the solid state. The complexes are stable towards oxidation, all except one have high spin electronic configuration. The oxidation potentials strongly depend on the two co-ligands. # 2002 Elsevier Science B.V. All rights reserved.

Borane‐Substituted Imidazol‐2‐ylidenes: Syntheses, Structures, and Reactivity☆

Reactions of the Lewis acids BH3 and BEt3 with trimethylimidazole (1) lead to the borane adducts 2a and 2b. Deprotonation of 2a with n-butyllithium results in the formation of the novel N-borane-substituted imidazol-2-ylidene anion 3a– whereas deprotonated 2b rearranges unexpectedly to the anionic compound 3b–. This can be transformed into the carbene–borane adduct 4 by methylation. The reaction of 3a– with [Mn(CO)5Br] yields the carbene complex 5. Surprisingly, 3a– attacks Fe(CO)5 at a carbon atom which leads to the iron acyl complex anion 6–. The compositions of the products follow from spectroscopic and analytical data and from X-ray structure analyses for Li(thp)+3a–, Li(thf)2+3b– and Li(thp)3+6–.

Tunable TACN/pyrazolate hybrid ligands as dinucleating scaffolds for metallobiosite modeling—dinickel(II) complexes relevant to the urease active site

Abstract Two new dinucleating pyrazole ligands HL1 and HL2 bearing appended 1,4-diisopropyl-1,4,7-triazacyclononane (iPr2TACN) side arms in the 3- and 5-positions have been synthesized by multi-step synthetic procedures. HL1 and HL2 differ by the length of the spacer between the pyrazole and the iPr2TACN, which is a novel means of tuning the metal–metal-separation in pyrazolate-based bimetallic compounds. Complexes [L1Ni2(H3O2)](ClO4)2 (5) and [L2Ni2(OH)](BPh4)2 (6b) have been characterized structurally, where the bridging H3O2 moiety in 5 is reactive and allows cooperative substrate transformations within the bimetallic pocket, while the tightly bound OH bridge in 6b is inert. Using the dinickel scaffold 5, reactivity relevant to the urease active site has been probed. Under anhydrous conditions, 5 reacts with parent and N-substituted urea to yield complexes 8a–c featuring N,O-bridging ureate anions. At high temperatures, 8a–c lose ammonia to give the cyanate-bridged species 10. This is able to bind a second cyanate at one of the nickel ions forming 11. Complexes with bridging acetate (7), O,O-bridging carbamate (9) and N,O-bridging methyl carbamate (12) have been studied for comparison, and mutual interconversions of the dinickel complexes have been investigated. Complexes 5, 6b, 7, 8a–c, 9, 11 and 12 have been characterized by X-ray crystallography.

Structural variation in transition-metal bispidine compounds.

The experimentally determined molecular structures of 40 transition metal complexes with the tetradentate bispyridine-substituted bispidone ligand, 2,4-bis(2-pyridine)-3,7-diazabicyclo[3.3.1]nonane-9-one [M(bisp)XYZ]n+; M = CrIII, MnII, FeII, CoII, CuII, CuI, ZnII; X, Y, Z = mono- or bidentate co-ligands; penta-, hexa- or heptacoordinate complexes) are characterized in detail, supported by force-field and DFT calculations. While the bispidine ligand is very rigid (N3...N7 distance = 2.933 +/- 0.025 A), it tolerates a large range of metal-donor bond lengths (2.07 A < sigma(M-N)/4 < 2.35 A). Of particular interest is the ratio of the bond lengths between the metal center and the two tertiary amine donors (0.84 A < M-N3/M-N7 < 1.05 A) and the fact that, in terms of this ratio there seem to be two clusters with M-N3 < M-N7 and M-N3 > or = M-N7. Calculations indicate that the two structural types are close to degenerate, and the structural form therefore depends on the metal ion, the number and type of co-ligands, as well as structural variations of the bispidine ligand backbone. Tuning of the structures is of importance since the structurally differing complexes have very different stabilities and reactivities.

Stabilization of Copper Dioxygen Compounds: Design, Synthesis, and Characterization

The design of a new type of binucleating ligand, which stabilizes end-on (μ-peroxo)dicopper(II) compounds (see picture) at ambient temperature, was achieved through molecular modeling.

Synthesis and Reactivity of Monoborylacetylene Derivatives

The catechol-substituted monoborylacetylenes 1a−d are obtained from the reaction of bis(diisopropylamino)borylacetylene with catechol derivatives and 2,2′-biphenol. The catalytic trimerization of 1a−d with [(η5-C5H5)Co(CO)2] yields isomeric mixtures of the triborylbenzene derivatives 2a,2a′, 2b,2b′, and 2c,2c′. The reaction of 2a,2a′ with mesityllithium provides the hexamesityl-substituted 1,3,5-triborylbenzene 2e. Hydroboration of 1a with catecholborane affords a mixture of 1,1-bis(1,3,2-benzodioxaborol-2-yl)ethene (3a) and trans-1,2-bis(1,3,2-benzodioxaborol-2-yl)ethene (4a). Hydroboration of 1a and 3a with one or two mol of HBCl2 and subsequent substitution of the chlorine atoms of the product with catechol leads in each case to the 1,1,1-trisborylmethane derivative 5a in 83 and 78% yield, respectively, which forms the tris(THF) adduct 5a(thf)3. Treatment of 5a with tBuLi yields 1,1,1-tris[di(tert-butyl)boryl]ethane 6a. [Co2(CO)8] reacts with 1a to give 3-(1,3,2-benzodioxaborol-2-yl)-1,2-bis(tricarbonylcobalta)tetrahedrane (9a). The new compounds have been characterized by NMR spectroscopy and mass spectrometry as well as by X-ray structure analyses for 1a, 3a, 5a, 5a(thf)3, and 9a.

A copper(I) oxygenation precursor in the entatic state: two isomers of a copper(I) compound of a rigid tetradentate ligand

Oxygenation of [CuI(L1)(NC-CH3)]+ (L1 = dimethyl 2,4-bis(2-pyridinyl)-3,7-diazabicyclo-[3.3.1]-nonane-9-on-1,5-dicarboxylate) leads to a relatively stable mu-peroxo-dicopper(II) product. The stability of this type of oxygenation product has been shown before to be the result of the square pyramidal geometry of L1; preorganization by a dinucleating ligand has been shown to increase the stability of the mu-peroxo-dicopper(II) compound. The structural data presented here indicate that destabilization of the copper(I) precursor is another important factor. There are two isomers of [CuI(L1)(NCCH3)]+; one is yellow, and the other is red. X-ray crystallography indicates that one pyridinyl donor is not coordinated in the yellow compound and that the red compound is 5-coordinate. In the light of the X-ray structure of the metal-free ligand and that of the corresponding copper(II) compound, it emerges that the ligand cavity is well suited for copper(II), whereas the copper(I) compounds are highly strained. This is supported by 1H NMR spectra of the copper(I) species where a fast dynamic process leads to line broadening and by electrochemical data, which indicate that the copper(II) products are exceptionally stable. Also presented are structural (copper(II)), electrochemical, and spectroscopic data (1H NMR, copper(I)) of the derivative [Cu(L2)(X)]n+ with a methyl substituent at the alpha-carbon atom of the two coordinated pyridinyl groups (L2 = dimethyl 2,4-bis(2-pyridinyl-6-methyl)-3,7-diazabicyclo-[3.3.1]-nonane-9-on-1,5-dicarboxylate). There are two structural forms of [CuII(L2)(X)]n+ (X = NCCH3, Cl), which depend on the steric demand of the fifth donor X. For both, van der Waals repulsion leads to a destabilization of the copper(II) products, and this is also evident from an increase in the reduction potential (-110 mV vs. -477 mV, Ag/AgNO3).

Reaktive π-Komplexe der elektronenreichen Übergangsmetalle. IV: Cyclisierungsreaktionen von t-Butylphosphaacetylen an π-Areneisen-Komplexen

Abstract The reaction of t-butylphosphaacetylene and bis(ethene)(toluene)iron or (toluene)(1-methylnaphthalene)iron yields t-butyl derivatives of three new complexes (toluene)(1,3-diphosphete)iron (4), (1,3-diphospholyl)(1,3-diphosphete)iron (5), and (1,3-diphospholyi)(1,2,4-triphospholyl)iron (6). 4 shows interesting redox properties.

Metal complexes of anionic 3-borane-1-alkylimidazol-2-ylidene derivatives

Addition of BH 3 ·thf to 1-alkylimidazoles (alkyl=methyl, butyl) and 1-methylbenzimidazole leads to BH 3 adducts, which are deprotonated by BuLi to yield the organolithium compounds (L)Li + ( 1b – d ) − . In the solid state (thf)Li + 1b − is dimeric. The acyl–iron complexes (thf) 3 Li + ( 3b , d ) − are formed from (thf)Li + ( 1b , d ) − and Fe(CO) 5 . (L)Li + ( 1a – c ) − react with [CpFe(CO) 2 X], however, the only complex obtained is [CpFe(CO) 2 1a ] (5a ). The analogous reaction of (L)Li + 1a − with the pentadienyl complex [(C 7 H 11 )Fe(CO) 2 Br] yields the corresponding iron compound 6a . Their compositions follow from spectroscopic data. Treatment of Cp 2 TiCl with (L)Li + 1a − leads to [Cp 2 Ti 1a ] ( 7a ), which could not be oxidized with PbCl 2 to give the corresponding Ti(IV) complex. The compounds [Li(py) 4 ] + 9a − and [Li(L) 4 ] + ( 10b – d ) − are obtained when (L)Li + 1 − are reacted with VCl 3 and ScCl 3 . The X-ray structure analysis of the vanadium complex reveals a distorted tetrahedron of the anion [V( 1a ) 4 ] − with two smaller and four larger CVC angles. The scandium compound [Li(dme) 2 + 10c − ] has a different structure: the distorted tetrahedron of the anion [Sc( 1c ) 4 ] − contains two larger (140.2 and 142.9°) and four smaller CScC angles (93.9–98.7°). This arrangement allows the formation of four bridging BHSc 3c,2e bonds to give an eight-fold coordination. The anion 10c − is formally a 16e complex.