Protic Ligands Research Articles

Imidazole Based Ruthenium(IV) Complexes as Highly Efficient Bifunctional Catalysts for the Redox Isomerization of Allylic Alcohols in Aqueous Medium: Water as Cooperating Ligand

Bis-allyl ruthenium(IV) complexes containing 1,3 azole β-N-H protic ligands [Ru(η3:η3-C10H16)Cl2L] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) (L = imidazole (1a), benzimidazole (1b)), and N-methylimidazole (1c) are highly active precatalysts in the redox isomerization of allylic alcohols into carbonyl compounds in aqueous medium and in the absence of base. A wide series of primary and secondary allylic alcohols can be isomerized into the corresponding carbonyl compounds. Remarkably, complex 1b has been found to be the most efficient catalyst reported to date for the isomerization of 1-octen-3-ol in water leading to a turnover frequency (TOF) value of 60 000 h–1. Moreover, catalyst 1b can be recycled remaining active up to seven cycles. Density functional theory (DFT) calculations give evidence that the hydroxo complexes derived from 1a–c species can be formed in aqueous solution and that they can act as the catalytic active species in a bifunctional catalyzed process. This study demonstrate that in water the participation of the β-N-H protic group of the 1,3-azole ligands in the bifunctional catalysis is not required, provided that a water molecule can act as cooperating ligand.

Tetranuclear stiboxanes (RSb)4O6, exhibiting an adamantane-type structure

Depolymerization reactions of organostibonic acids with protic ligands have been investigated. Reaction of arylstibonic acids with 8-hydroxyquinoline (8-HQ), or {2-[1H-pyrazol-5(3)-yl]naphthalene-1-ol} (H(2)naphpz) in a 1 : 1 stoichiometry in refluxing toluene affords adamantane-like L(4)(RSb)(4)O(6) clusters [(p-XC(6)H(4)Sb)(4)(O)(6)(Q)(4)] (where X = Cl (1), Br (2), QH = 8-hydroxyquinoline), [(p-ClC(6)H(4)Sb)(4)(O)(6)(Hnaphpz)(4)]·H(2)naphpz (3) and [(p-Br-C(6)H(4)Sb)(4)(O)(6)(Hnaphpz)(4)](2)·H(2)naphpz (4). Further a tetrameric organoantimony oxo cluster, L(4)(RSb)(4)O(4) [(p-ClC(6)H(4)Sb)(4)(O)(4)(naphpz)(4)] (5) has also been isolated as a side product in the reaction of arylstibonic acid with naphthylphenolic pyrazole. Interestingly 1-4 structurally resemble the dimeric form of the antimony oxide Sb(2)O(3) and its mineral senarmontite.

Bis(imidate)palladium(ii) complexes with labile ligands. Mimics of classical precursors?

A novel synthetic route to prepare palladium(II) precursor analogous of classical [Pd(Cl)(2)(solvent)(2)] has been developed. Just stirring Pd(3)(AcO)(6) in dimethyl sulfide at room temperature, in the stoichiometric presence of protic imidate ligands, resulted in the precipitation of the desired complexes [Pd(imidate)(2)(SMe(2))(2)] (imidate = succinimidate (succ) 1, phthalimidate (phthal) 2, maleimidate (mal) 3, saccharinate (sac) 4 or glutarimidate (glut) 5). The new complexes are very soluble in common solvents and have been fully characterized, including an X-ray diffraction analysis of 2. Analogous reactions with succinimide in acetonitrile or dimethylsulfoxide produced [Pd(succinimidate)(2)(solvent)(2)] (6 and 7, respectively) as off-white powders. Thermal decomposition of 6 produces a new species 6* with bridging imidate ligands that can be formulated as a trimer similar to Pd(3)(AcO)(6). The usefulness of 1-5 as precursors has been tested by reactions against monodentated neutral donor ligands, PPh(3) (a compounds), or pyridine (py, b compounds), to produce ten new derivatives of the general formula trans-[Pd(imidate)(2)(L)(2)]. The single-crystal structures of compounds 2a, 3a, 4a, 4a', 5a and 4b have also been established, allowing an interesting molecular and supramolecular structural discussion. A cis-conformation was induced when the bidentate chelate ligand 1,2-bis(diphenylphosphino)benzene (dppb, c compounds) was made to react with 1-5. Structural characterization by X-ray diffraction of complex 2c confirmed the proposed formula. Catalytic activity in Suzuki-Miyaura cross-coupling of aryl bromides and benzyl bromides with aryl boronic acids has been tested.

Divergent Syntheses of Copper−Indium Bimetallic Single-Source Precursors via Thiolate Ligand Exchange

Several copper-indium bimetallic single-source precursors (SSPs 2-9) have been prepared efficiently by exchange reactions of (Ph(3)P)(2)CuIn(SEt)(4) (1) with protic ligands. This divergent approach has been successfully applied on multigram scales to produce nearly quantitative yields of known and newly reported SSPs. The former group features the previously difficult target (Ph(3)P)(2)CuIn(SePh)(4) (2), and the latter includes (Ph(3)P)(2)CuIn(SEt)(2)(SePh)(2) (3), the first SSP to incorporate both sulfur and selenium within a single copper-indium bimetallic complex.

Effect of second-sphere coordination 12. Adduct formation behavior of crown ethers with ammine complexes of ruthenium containing a protic ligand of other type than ammine

Adduct formation of pentaammineruthenium complexes involving a different type of protic ligand, such as imidazole, was investigated for a series of crown ethers with different ring size. Changes in redox potential and in absorption spectra of the complex were measured on addition of crown ether to the complex solution. The magnitude of the change in both properties is dependent on the ring size of crown ethers. 1H-NMR spectra of the complex were measured in the presence of crown ethers in order to elucidate hydrogen bonding sites. The chemical shifts of NH proton of imidazol and ammine protons were measured at various concentrations of crown ethers. Adduct formation was discussed based on the features of dependences of those chemical shifts on crown ether concentration.

Reactivity Towards Acidic Protic Ligands of Cyclopalladated Di‐μ‐hydroxo Complexes

AbstractThe dinuclear hydroxo complexes [{Pd(μ‐OH)(C∧N)}2] [C∧N = 2‐(2‐pyridyl)phenyl (Phpy) (I), 7,8‐benzoquinolyl (Bzq) (II) and 2‐(2‐oxazolinyl)phenyl (Phox) (III)] react in a 1:2 molar ratio with a wide variety of protic electrophiles H(L∧L) bearing different sets of donor atoms (L∧L = O∧O or O∧N) to give the mononuclear neutral palladium(II) derivatives with the general formula [Pd(L∧L)(C∧N)] [O∧O = salicylaldehydate (sal) (1), acetylacetonate (acac) (2) and benzoylacetonate (bzac) (3); O∧N = N‐phenylsalicylaldiminate (N‐Phsal) (4), N‐p‐chlorophenylsalicylaldiminate (N‐pClPhsal) (5), 2‐pyrrolecarbaldeydate (2‐pcal) (6), 8‐hydroxyquinolinate (oxin) (7)]. Structural characterisation of complexes I1, I3, I6, II4, II5 and III5 by X‐ray diffraction confirmed the proposed formulae. Dinuclear complexes [{Pd(μ‐N∧S)(C∧N)}2] [N∧S = 2‐pyridinethiolate (spy)] (8) were obtained when treating complexes I–III with 2‐mercaptopyridine in the same molar ratio. A related process takes place when the three precursors react with ammonium O,O′‐diethyldithiophosphate under mild conditions and complexes [Pd{S(S)P(OEt)2}(C∧N)] (I9–III9) are obtained. Deprotonation of the secondary amine Et2NH by complexes I–III in the presence of carbon disulfideleads to the corresponding dithiocarbamate complexes [Pd(S2CNEt2)(C∧N)] (I10–III10). The new complexes were characterised by analytical and spectroscopic techniques (IR and 1H NMR).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Design of Bifunctional Molecular Catalysts for Hydrogenation of Polar Functionalities

Recent progress in our bifunctional molecular catalysts for the reductive transformation of functional groups with electronically localized C-O bond (polar functionalities) using molecular hydrogen(H2) is described. The newly developed Cp*Ru(PN) complexes have proven to be efficient catalysts for the hydrogenation of a range of polar functionalities. The structural modification of the protic amine ligand improves the catalytic performance and allows efficient access to various chiral compounds of synthetic value.

Structural and Spectroscopic Study of Reactions between Chelating Zinc-Binding Groups and Mimics of the Matrix Metalloproteinase and Disintegrin Metalloprotease Catalytic Sites: The Coordination Chemistry of Metalloprotease Inhibition

To understand the coordination chemistry of zinc-binding groups (ZBGs) with catalytic zinc centers in matrix metalloproteinases (MMPs) and disintegrin metalloproteases (ADAMs), we have undertaken a model compound study centered around tris(3,5-methylphenypyrazolyl)hydroboratozinc(II) hydroxide and aqua complexes ([Tp(Ph,Me)ZnOH] and [Tp(Ph,Me)Zn(OH2)]+, respectively, wherein (Tp(Ph,Me))- = hydrotris(3,5-methylphenylpyrazolyl)borate) and the products of their reactions with a class of chelating Schiff's base ligands. The results show that the protic ligands, HL (HL = N-propyl-1-(5-methyl-2-imidazolyl)methanimine (5-Me-4-ImHPr), N-propyl-1-(4-imidazolyl)methanimine (4-ImHPr), and N-propyl-1-(2-imidazolyl)methanimine (2-ImHPr)), react with [Tp(Ph,Me)ZnOH] and give products with the general formula [Tp(Ph,Me)ZnL], whereas reactions with neutral aprotic ligands, L' (L' = N-propyl-1-(1-methyl-2-imidazolyl)methanimine (1-Me-2-ImPr) and N-propyl-1-(2-thiazolyl)methanimine (2-TaPr)), yield the corresponding [Tp(Ph,Me)ZnL]+ complexes. Although the phenol group of N-propyl-1-(2-hydroxyphenyl)methanimine (2-HOPhPr) is protic, this ligand forms a cationic four-coordinate complex containing an intraligand hydrogen bond. The solid-state structures of these complexes were determined by single-crystal X-ray diffraction, and the results showed that the protic ligands form five-membered chelates of the Zn2+ ion. All ligands displace the aqua ligand in [Tp(Ph,Me)Zn(OH2)]+ to yield complexes having 1H NMR spectra consistent with the formation of five membered chelates. The 1H resonance frequencies of the chelating ligands typically shift upfield upon coordination to the zinc center, due to ring current effects from the pendant phenyl groups of the (Tp(Ph,Me))- ligand. Thus, the 1H NMR spectra provide a convenient and sensitive means of tracking the solution reactions by titration. The resulting series of spectra showed that the stabilities of the chelates in solution depend on the propensity of the ligands to deprotonate upon chelation of the zinc center. The behaviors of these bidentate ZBGs provide insight into the structural and electronic factors that contribute to the stabilities of inhibited MMPs and ADAMs and suggest that the proton acidity of the coordinated ZBG may be a crucial criterion for inhibitor design.

Effect of second-sphere coordination 11. Selectivity in adduct formation of polypyridineruthenium(II) complexes with crown ethers

Adduct formation of polypyridineruthenium(II) complexes involving a different type of protic ligand was investigated for a series of crown ethers with different ring size. Changes in redox potential and in absorption spectra of the complex were measured on addition of crown ether to the complex solution. The magnitude of the change in both properties is dependent on the ring size of crown ethers. Chemical shifts of protic ligand of the complexes were measured at various concentrations of crown ethers. The stability constants of crown ether adduct were determined from the dependences of the chemical shift on crown ether concentration. It is found from their stability constants that the individual complex shows the different selectivity in adduct formation with crown ethers.

DFT Study of Solvent Coordination Effects on Titanium-Based Epoxidation Catalysts. Part Two: Reactivity of Titanium Hydroperoxo Complexes in Ethylene Epoxidation

Density functional theory has been used to study the effects of solvent coordination on the reactivity of titanium hydroperoxo complexes in the epoxidation of the model olefin ethylene. Complexes possessing a variety of coordination environments have been modeled with unconstrained single coordination sphere clusters using a B3LYP/ECP methodology. The Gibbs free energy of activation for ethylene epoxidation with a nonligated titanium hydroperoxo species is 25.7 kcal/mol. Addition of a single solvent ligand to the titanium center has little effect on the activation barrier unless that ligand participates in a hypervalent ω bond with the proximal oxygen (Oα) of the hydroperoxo moiety; in that case, the ω bond inhibits the heterolytic cleavage of the Ti−Oα bond and increases the activation barrier for transfer of Oα to ethylene by 6−7 kcal/mol. Hydrogen bonding of the hydroperoxo moiety with a protic solvent ligand via a monodentate five-membered ring structure does not enhance the reactivity of the titanium hydroperoxo complex. The presence of two solvent ligands on the titanium center increases the activation barrier 11−16 kcal/mol by sterically hindering the attack of ethylene at Oα. The activation barrier is not affected by the presence of a hydrogen-bonded protic solvent molecule bridging the hydroperoxo moiety and a protic ligand on titanium. NBO analysis indicates that changes in the electrophilicity of the peroxo group caused by solvent coordination do not control the observed reactivity of the titanium hydroperoxo complexes. The activation barriers for ethylene epoxidation with titanium hydroperoxo, methylperoxo, and trifluoromethylperoxo oxidants decrease with increasing positive charge on the atom bonded to the distal peroxo oxygen (Oβ) through stabilization of the developing negative charge on Oβ. Comparison of the Gibbs activation barriers for the formation of the titanium hydroperoxo intermediate and its subsequent reaction with ethylene indicates that the oxygen transfer step is the rate-determining step for the overall epoxidation mechanism.

DFT Study of Solvent Coordination Effects on Titanium-Based Epoxidation Catalysts. Part One: Formation of the Titanium Hydroperoxo Intermediate

Density functional theory has been used to study the effects of solvent coordination on the structure and formation of the titanium hydroperoxo species believed to be the active intermediate in oxidation reactions with Ti(IV)−H2O2 catalytic systems. Titanium hydroperoxo intermediates possessing a variety of coordination environments have been modeled with unconstrained single coordination sphere clusters using a B3LYP/ECP methodology. Titanium hydroperoxo intermediates bearing one or two solvent ligands may assume several degenerate structures with the hydroperoxo moiety coordinated to titanium in a monodentate or bidentate manner. Hydrogen bonding of the hydroperoxo moiety with a protic solvent ligand via a monodentate five-membered ring structure does not confer any significant additional stabilization to the titanium hydroperoxo intermediate. The presence of a single solvent ligand on the titanium center lowers the Gibbs free energy of activation for the formation of titanium hydroperoxo intermediates by 4−8.5 kcal/mol. Hydrogen-bonded protic molecules can facilitate proton transfer from a hydrogen peroxide ligand on titanium and thereby lower the Gibbs activation barrier for intermediate formation by 5−6 kcal/mol. NBO analysis provides an enriched understanding of how solvent ligands alter structural and electronic properties and reduce activation barriers. Consideration of thermodynamic and kinetic results indicates that the most abundant titanium hydroperoxo intermediates formed under liquid-phase reaction conditions will most likely possess a single solvent ligand on titanium and possibly a hydrogen-bonded protic solvent molecule.

Potentially Tridentate Ligands in the Synthesis of Methyl- and Acetylpalladium(II) Complexes

The two protic tridentate ligands HNN′O (1) and HNN′S (2) were treated with [(COD)Pd(Me)Cl], and the corresponding methylpalladium(II) complexes [Pd(η2-HNN′)(Me)Cl] (1a and 2a) were isolated. The neutral ligands in these complexes coordinated the metal in an NN′ bidentate mode, through the pyridine and imine nitrogens, excluding the O or S atom from the coordination. This behavior was confirmed by X-ray diffraction analysis of the dichloro complex of 1, [Pd(η2-HNN′)Cl2] (1c). Complexes 1a and 2a easily transformed quantitatively into the corresponding three-coordinate complexes [Pd(η3-NN′X)(Me)] (1b: X = O, 2b: X = S) in basic medium, with elimination of HCl. The complexes 1a and 1b were treated with PPh3 and P(CD3)3, respectively, resulting in 1e and 1h; 1a reacted further with a double quantity of PPh3 to give rise to the diphosphane complex [trans-(PPh3)2Pd(Me)Cl] (1g) and free 1. The three-coordinate complexes 1b and 2b were subjected to CO bubbling at room temperature. In the case of 1b, acylation of the ligand on the phenolic oxygen was observed, with the formation of an organic acetate and release of palladium black. In the case of 2b, besides the acylated ligand on the thiolic sulfur, the acetylpalladium(II) complex [Pd(η3-NN′S)(MeCO)] (2c) was isolated. The X-ray structures of 1c·THF, 1b·1/2C6H6O2, 2b, 2c·1/2CH2Cl2, and 1g have been solved. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Acetonitrile complexes of triply charged metal ions: are ligated trications intrinsically more prone to charge reduction than dications?

The development of electrospray has enabled the generation of gas-phase solvated multiply charged metal ions. Complexes involving both protic ligands (water, alcohols) and aprotic ones (e.g., ketones, DMSO, acetonitrile) were found for dications, but trications were compatible with aprotic ligands only. Here, to probe this difference in stability, acetonitrile complexes of metal trications were formed by ESI and collisionally fragmented. Proton transfer, electron transfer, and heterolytic cleavage channels previously found for dications were observed. Characteristic sizes for these processes suggest an intrinsic gap in the stability of dication and trication complexes against proton transfer, but not against electron transfer.

Gas-phase metal trications in protic solvent complexes.

In the first observation of triply charged metal ions microsolvated by protic ligands, gas-phase complexes of 11 different metal trications with diacetone alcohol were generated using electrospray ionization. These species were found to exist for at least five to eight ligands, depending on the metal. For smaller sizes, charge reduction to dications occurs spontaneously via dissociative proton transfer. This is the largest minimum size reported for any multiply charged metal cation/ligand pair.

Reactions with the technetium and rhenium carbonyl complexes (NEt 4) 2[MX 3(CO) 3]. Synthesis and structure of [Tc(CN-Bu t) 3(CO) 3](NO 3) and (NEt 4)[Tc 2(μ-SCH 2CH 2OH) 3(CO) 6

Starting from [MO 4] − (M = Tc, Re) the low-pressure (1 atm.) synthesis of the important starting material (NEt 4) 2[MCl 3(CO) 3] could be improved in terms of yield and time. These M I complexes were used for the preparation of compounds containing the “ fac-M(CO) 3” moiety under ambient conditions. The substitution with the isocyanide ligand CN-Bu t is described. In the case of rhenium, IR spectroscopical investigations in the CO stretching region allowed the observation of the stepwise formation of [Re(CN-Bu t) 3(CO) 3] +. It could be demonstrated that 99Tc NMR spectroscopy allows similar investigations, although not all types of intermediates could be detected. The structure of the complex [Tc(CN-Bu t) 3(CO) 3](NO 3) was elucidated by X-ray diffraction analysis. Substitution with the protic ligand 2-mercaptoethanol HSCH 2CH 2OH revealed similar behaviour. Two intermediates were observed with IR spectroscopical methods. The final product was the dimer (NEt 4)[Tc 2(μ-SCH 2CH 2OH) 3(CO) 6], the structure of which was determined by X-ray diffraction.

An Organometallic Methodology Affording Dinuclear Copper(I) Complexes.

The metalation of protic dinucleating ligands using [Cu5Mes5] allowed us to avoid any disproportionation and obtain dinuclear copper(I) complexes appropriate for reactivity studies.

Iron(II)-.alpha.-Amino Acid Complexes from Nonprotic Solutions: Reactions of [Fe(Mes)2(phen)] with .alpha.-Amino Acids and the Structures of Bis(L-Prolinato)(1,10-phenanthroline)iron(II) and Bis(D-prolinato)(1,10-phenanthroline)iron(II)

The arylation of FeCl 2 (THF) 1.5 in THF with MesMgBr (Mes=1,3,5-Me 3 C 6 H 2 ) in the presence of 9,10-phenanthroline (phen) led to the crystalline [Fe(phen)(Mes) 2 ], 1, which contains a tetrahedral iron(II) σ-bonded to two aryl groups. They can be removed by a variety of protic ligands under aprotic conditions. The reaction of 1 with amino acids AH led to the isolation of monomeric iron(II)-amino acid complexes [Fe(phen)(A) 2 ]

Second‐Sphere Coordination–a Novel Rǒle for Molecular Receptors

AbstractAlthough it has been appreciated for most of this century that the coordinating influence of many transition metal complexes extends beyond their covalently‐bonded first‐sphere ligands to non‐covalently bound chemical species in the so‐called second‐sphere, it is only recently that the fundamental importance of the phenomenon has been recognized and researched in its most general context. Rapid progress has been possible in this relatively new area of supramolecular chemistry by appealing to the symbiotic relationship that exists between X‐ray crystallographic investigations and synthetic work. The gamut of non‐covalent bonds, including electrostatic forces, hydrogen bonding, charge transfer, and van der Waals interactions are available for exploitation in choosing or designing suitable molecular receptors for particular transition metal complexes. Crown ethers are the synthetic macrocycles par excellence for forming adducts with neutral and cationic complexes carrying protic ligands (NH3, H2O, CH3CN, etc.) in their first coordination spheres. Anionic complexes with electron‐rich first‐sphere ligands (e.g. CN⊖) form adducts with polyammonium macrocyclic receptors. In both situations, hydrogen bonds and electrostatic interactions provide the dominant sources of supramolecular stabilization. Spectroscopic studies (UV, IR, and NMR) reveal that the structural integrity of the adducts is usually maintained in solution. The transition metal complexes can be organometallic as well as inorganic. In complexes supporting organic ligands, such as 1,5‐cyclooctadiene, norbornadiene, cyclopentadiene, 2,2′‐bipyridine, and trimethylphosphane, the opportunity exists to match their steric and electronic characteristics with appropriate binding sites in the molecular receptor. With these ligands, the weaker categories of non‐covalent bonding, e.g. charge transfer and van der Waals interactions, assume considerable importance. The phenomenon is also observable with naturally‐occurring receptors such as the cyclodextrins and polyether antibiotics. Moreover, it extends beyond the territory of transition metal complexes to main group complexes such as ammonia‐borane. As far as applications are concerned, it finds expression in areas as diverse as separation science and drug delivery systems.

Synthesis, reactions and characterisation of some new β-ketoester boroacetates

The reactions of methyl esters of aroyl pyruvic acids (aroyl = benzoyl,p-chlorobenzoyl,p-bromobenzoyl andp-methylbenzoyl) with boric acid in acetic anhydride solution yield derivatives of the general formula (OAc)2B [OC(R)CHCOCOOMe] (R = C6H5,p-ClC6H4,p-BrC6H4 andp-CH3C6H4). The reactions of these derivatives with protic ligands such as glycols,o-aminophenol ando-aminothiophenol have also been carried out. These derivatives have been characterised by elemental analysis and molecular weight measurements. The tetra-coordination around central boron atom has been established byir,1Hnmr and 11 Bnmr spectral evidence.

Kinetics and mechanism of ligand substitution in tetrahedral zinc complexes

Zinc(II) complexes are of major importance for biological systems. Compared with other divalent transition metal complexes there is relatively little information on the kinetics and mechanism of ligand substitution in zinc(II) complexes. This is probably due to the fact stabilization effects. Hence, zinc(II) systems are in general kinetically labile and not coloured, which makes monitoring more difficult. Ligand substitution in systems such as (1) ZnA 2 + 2HB ⇄ ZnB 2 + 2HA can be easily followed spectrophotometrically if the absorption of ZnA 2 in the UV/VIS range is stronger than that of HA, HB, and ZnB 2. This is so for the tetrahedral complexes ZnA 2 ≙ I ≙ Zn(X-sal-R) 2 ≙ bis-(N-alkylsalicylaldiminato)-Zn(II), ZnA 2 ≙ II ≙ Zn(HMDP) 2 ≙ bis-(hexamethyldipyrromethenido)- Zn(II), and for ligands HB such as acetylacetone ≙ Hacac, even if present in large excess. The kinetics of ligand substitution in complexes I and II were studied by SF spectrophotometry ( I) and normal spectrophotometry ( II) in organic solvents. Mechanistic information was obtained through variation of the substituent X, of the alkyl group R and of the nature of the attacking ligand HB. ▪ In protic solvents ligand substitution according to (1) follows the general rate law (2), in which K s describes a ligand independent pathway induced by the solvent: v = k obs * [complex] = (k s + k HB [HB]) * [complex] (2) The results obtained can be summarized as follows: 1. X-ray structures of complexes Zn(X-sal-R) 2 with X = H and R = Et, iPr, nBu and with X 4 = OMe and R = nPr prove tetrahedral coordination geometry in the solid state. 2. Complexes Zn(X-sal-R) 2 react much faster than complexes Zn(HMDP) 2, for which k s = O. 3. For the salicylaldiminato complexes Zn(X-sal-R) 2 the relative contributions of k s and k HB · [HB] to k obs as well as the size of k obs are governed by the nature of R. 4. The kinetics effect of substituents X in the 5-position is not very significant in protic solvents. 5. Variation of the nature of HB clearly reveals the associative character of the ligand pathway k HB.