Pyrazolylborate-zinc alkoxide complexes. 3. Acid-base reactions

The alkoxides TpPh,MeZn-OR (R = Me, Et, i-Pr) undergo acid-base reactions with all hydrogen compounds whose acidity is higher than that of the corresponding alcohol ROH. Thus, anion exchange occurs with the common acids acetic acid, acetohydroxamic acid, acetylacetone, phenol, and ethylmercaptan. Alkoxide exchange is observed using methanol, ethanol, and trifluoroethanol. With the NH acids cyanamide, trifluoroacetamide, and pyrazoles, the corresponding anions are attached to zinc, and likewise beta- and gamma-lactams, a thiazolidinedione, and the cyclic sulfimide saccharin are deprotonated. Of the CH acids acetonitrile forms the Tp*Zn-cyanomethanide. Acetone is deprotonated by the cyanomethanide complex and incorporated as the Tp*Zn-beta-ketoiminate.     

Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes.

The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhC&tbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2beta, and a minor alpha-agostic rotamer 2alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R(2)C=CMe) [Tp' = Tp(Me2), R(2) = Me (3); Tp' = Tp(Me2,4Cl), R(2) = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta-agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta-agostic methylene complex 5AR-CSbeta and a minor alpha-agostic species 5AR-CSalpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2beta/2alpha = 4.0 +/- 0.1 at 193 K, DeltaG(o)(193) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-)(1), and DeltaS(o) = -27 +/- 1 J K(-)(1) mol(-)(1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG(2)= 47.5 +/- 2.5, DeltaH= 58.8 +/- 2.5 kJ mol(-)(1), and DeltaS = 59.0 +/- 10 J K(-)(1) mol(-)(1)). Upon selective deuteration of the beta-methyl protons in Tp(Me2)NbCl[CH(CD(3))(2)](PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the alpha-agostic rotamer is observed (K = 2-d(6)beta/2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(o)(193) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-)(1) and DeltaS(o)= -34 +/- 2 J K(-)(1) mol(-)(1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl(i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta-agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH(2)Me)(HC=CMe) is alpha-agostic only and that Tp(5)(-)(Me)NbCl(CH(2)Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a beta-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction.     

Pyrazolylborate-zinc alkoxide complexes. 2. Solvolytic chemistry

The methoxides TpPh,MeZn-OMe and TpCum,MeZn-OMe were tested for their reactivity toward substrates that are hydrolytically cleaved with Tp*Zn-OH complexes. They do not induce the cleavage of nonactivated esters, phosphoesters, lactones, or lactams. They cleave the P-O-P linkage of tetraalkylpyrophosphates, but not the C-O-C linkage of dialkyl pyrocarbonates. Transesterification of esters and phosphoesters occurs when they are activated as p-nitrophenolates. The most facile cleavage occurs for thiolate functions present in dithioesters, thiolactones, and trithiocarbonates. These findings indicate that, while the leaving group properties of the methoxide unit are essential, it is the strength of the resulting zinc-substrate bonds that decides upon the occurrence or nonoccurrence of the cleavage reactions.     

Synthesis and characterisation of second-generation metallodithiolene complexes of the type [Tp*ME(dithiolene)](M=Mo, W; E=O, S) and a novel 'organoscorpionate' complex of tungsten

Paramagnetic, chalcogenido-M(v) dithiolene complexes, [Tp*ME{S2C2(CO2Me)2}][M=Mo, E=O, S; M=W, E=O, S; Tp*=hydrotris(3,5-dimethylpyrazol-1-yl)borate] are generated in the reactions of dimethyl acetylenedicarboxylate (DMAC) and the sulfur-rich complexes NEt4[Tp*MoS(S4)] and NEt4[Tp*WS3]; the oxo complexes result from hydrolysis of the initial sulfido products. As well, a novel 'organoscorpionate' complex, [W{S2C2(CO2Me)2}{SC2(CO2Me)2-Tp*}], has been isolated from the reactions of NEt4[Tp*WS3] with excess DMAC. Complexes , and have been isolated and characterised by microanalytical, mass spectrometric, spectroscopic and (for and) X-ray crystallographic techniques. Complexes and have been partially characterised by mass spectrometry and IR and EPR spectroscopy. Six-coordinate, distorted-octahedral contains a terminal sulfido ligand (W=S=2.108(3)A), a bidentate dithiolene ligand (S-Cav=1.758 A, C=C=1.332(10)A) and a fac-tridentate Tp* ligand. Seven-coordinate contains a planar, bidentate dithiolene ligand (S-Cav=1.746 A, C=C=1.359(5)A) and a novel pentadentate 'organoscorpionate' ligand formed by the melding of DMAC, sulfido and trispyrazolylborate units. The latter is coordinated through two pyrazolyl N atoms (kappa2-N,N') and a tridentate kappa3-S,C,C' unit appended to N-beta of the third (uncoordinated) pyrazolyl group. The second-generation [Tp*ME(dithiolene)] complexes represent a refinement on first-generation [Tp*ME(arene-1,2-dithiolate)] complexes and their synthesis affords an opportunity to compare and contrast the electronic structures of true vs. pseudo-dithiolene ligands in otherwise analogous complexes.     

Crystal structures and solution behavior of paramagnetic divalent transition metal complexes (Fe, Co) of the sterically encumbered tridentate macrocycles 1,4,7-R3-1,4,7-triazacyclononane: coordination numbers 5 (R = i-Pr) and 6 (R = i-Bu)

The coordination chemistry of the sterically hindered macrocyclic triamines, 1,4,7-R3-1,4,7-triazacyclononane (R = i-Pr, i-Pr3tacn, and R = i-Bu, i-Bu3tacn) with divalent transition metals has been investigated. These ligands form a series of stable novel complexes with the triflate salts MII(CF3SO3)2 (M = Fe, Co, or Zn) under anaerobic conditions. The complexes Fe(i-Pr3tacn)(CF3SO3)2 (2), [Co(i-Pr3tacn)(SO3CF3)(H2O)](CF3SO3) (3), [Co(i-Pr3tacn)(CH3CN)2](BPh4)2 (4), Zn(i-Pr3tacn)(CF3SO3)2 (5), [Fe(i-Bu3tacn)(CH3CN)2(CF3SO3)](CF3SO3) (6), Fe(i-Bu3tacn)-(H2O)(CF3SO3)2 (7), and Co(i-Bu3tacn)(CF3SO3)2 (8) have been isolated. The behavior of these paramagnetic complexes in solution is explored by their 1H NMR spectra. The solid-state structures of four complexes have been determined by X-ray single-crystal crystallography. Crystallographic parameters are as follows. 2: C17H33F6FeN3O6S2, monoclinic, P2(1)/n, a = 10.895(1) A, b = 14.669(1) A, c = 16.617(1) A, beta = 101.37(1) degrees, Z = 4. 3: C17H35CoF6N3O7S2, monoclinic, P2(1)/c, a = 8.669(2) A, b = 25.538(3) A, c = 12.4349(12) A, beta = 103.132(13) degrees, Z = 4. 6: C24H45F6FeN5O6S2, monoclinic, P2(1)/c, a = 12.953(6) A, b = 16.780(6) A, c = 15.790(5) A, beta = 96.32(2) degrees, Z = 4. 7: C20H41F6FeN3O7S2, monoclinic, C2/c, a = 22.990(2) A, b = 15.768(2) A, c = 17.564(2) A, beta = 107.65(1) degrees, Z = 8. The ligand i-Pr3tacn leads to complexes in which the metal ions are five-coordinate, while it's isobutyl homologue affords six-coordinate complexes. This difference in the stereochemistries around the metal center is attributed to steric interactions involving the bulky alkyl appendages of the macrocycles.     

Synthesis, structures, and reactivity of yttrium alkyl and alkynyl complexes with mixed Tp(Me2)/Cp ligands

The structurally characterized Tp(Me2)-supported rare earth metal monoalkyl complex (Tp(Me2))CpYCH(2)Ph(THF) (1) was synthesized via the salt-metathesis reaction of (Tp(Me2))CpYCl(THF) with KCH(2)Ph in THF at room temperature. Treatment of 1 with 1 equiv of PhC≡CH under the same conditions afforded the corresponding alkynyl complex (Tp(Me2))CpYC≡CPh(THF) (2). Complex 1 exhibits high activity toward carbodiimides, isocyanate, isothiocyanate, and CS(2); treatment of 1 with such substrates led to the formation of a series of the corresponding Y-C(benzyl) σ-bond insertion products (Tp(Me2))CpY[(RN)(2)CCH(2)Ph] (R = (i)Pr(3a), Cy(3b), 2,6-(i)Pr-C(6)H(3)(3c)), (Tp(Me2))CpY[SC(CH(2)Ph)NPh] (4), (Tp(Me2))CpY[OC(CH(2)Ph)NPh] (5), and (Tp(Me2))CpY(S(2)CCH(2)Ph) (6) in 40-70% isolated yields. Carbodiimides and isothiocyanate can also insert into the Y-C(alkynyl) σ bond of 2 to yield complexes (Tp(Me2))CpY[(RN)(2)CC≡CPh] (R = (i)Pr(7a), Cy(7b)) and (Tp(Me2))CpY[SC(C≡CPh)NPh] (9). Further investigation results indicated that 1 can effectively catalyze the cross-coupling reactions of phenylacetylene with carbodiimides. However, treatment of o-allylaniline with a catalytic amount of 1 gave only the benzyl abstraction product (Tp(Me2))CpY(NHC(6)H(4)CH(2)CH═CH(2)-o)(THF) (10), without observation of the expected organic hydroamination/cyclization product. All of these new complexes were characterized by elemental analysis and spectroscopic properties, and their solid-state structures were also confirmed by single-crystal X-ray diffraction analysis.     

Synthesis of U(IV) imidos from Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) by extrusion of bibenzyl

Addition of organic azides, N(3)R (R = 2,4,6-trimethylphenyl (Mes), phenyl (Ph), 1-adamantyl (Ad)), to a solution of the uranium(III) alkyl complex, Tp*(2)U(CH(2)Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) (1), results in the formation of a family of uranium(iv) imido derivatives, Tp*(2)U(NR) (2-R). Notably, these complexes were synthesized in high yields by coupling of the benzyl groups to form bibenzyl. The uranium(IV) imido derivatives, 2-Mes, 2-Ph, and 2-Ad, were all characterized by both (1)H NMR and IR spectroscopy, and 2-Mes and 2-Ad were also characterized by X-ray crystallography. In the molecular structure of 2-Mes, typical κ(3)-coordination of the Tp* ligands was observed; however in the case of 2-Ad, one pyrazole ring of a Tp* ligand has rotated away from the metal centre, forcing a κ(2)-coordination of the pyrazoles. This results in a uranium-hydrogen interaction with the Tp* B-H. Treating these imido complexes with para-tolualdehyde results in multiple bond metathesis, forming the terminal uranium(IV) oxo complex, Tp*(2)U(O), and the corresponding imine.     

Preparation of mononuclear and dinuclear Rh hydrotris(pyrazolyl)borato complexes containing arenethiolato ligands and conversion of the mononuclear complexes into dinuclear Rh-Rh and Rh-Ir complexes with bridging arenethiolato ligands

Reactions of [Tp*Rh(coe)(MeCN)](; Tp*= HB(3,5-dimethylpyrazol-1-yl)(3); coe = cyclooctene) with one equiv. of the organic disulfides, PhSSPh, TolSSTol (Tol = 4-MeC(6)H(4)), PySSPy (Py = 2-pyridyl), and tetraethylthiuram disulfide in THF at room temperature afforded the mononuclear Rh(III) complexes [Tp*Rh(SPh)(2)(MeCN)](3a), [Tp*Rh(STol)(2)(MeCN)](3b), [Tp*Rh(eta(2)-SPy)(eta(1)-SPy)](6), and [Tp*Rh(eta(2)-S(2)CNEt(2))(eta(1)-S(2)CNEt(2))](7), respectively, via the oxidative addition of the organic disulfides to the Rh(I) center in 1. For the Tp analogue [TpRh(coe)(MeCN)](2, Tp = HB(pyrazol-1-yl)(3)), the reaction with TolSSTol proceeded similarly to give the bis(thiolato) complex [TpRh(STol)(2)(MeCN)](4) as a major product but the dinuclear complex [[TpRh(STol)](2)(micro-STol)(2)](5) was also obtained in low yield. Complex 3 was treated further with the Rh(III) or Ir(III) complexes [(Cp*MCl)(2)(micro-Cl)(2)](Cp*=eta(5)-C(5)Me(5)) in THF at room temperature, yielding the thiolato-bridged dinuclear complexes [Tp*RhCl(micro-SPh)(2)MCp*Cl](8a: M = Rh, 8b: M = Ir). Dirhodium complex [TpRhCl(micro-STol)(2)RhCp*Cl](9) was obtained similarly from 4 and [(Cp*RhCl)(2)(micro-Cl)(2)]. Anion metathesis of 8a proceeds only at the Rh atom with the Cp* ligand to yield [Tp*RhCl(micro-SPh)(2)RhCp*(MeCN)][PF(6)](10), when treated with excess KPF(6) in CH(2)Cl(2)-MeCN. The X-ray analyses have been undertaken to determine the detailed structures of 3b, 4, 5, 6, 7, 8a, 9, and 10.     

Functionalization of carbon dioxide and carbon disulfide using a stable uranium(III) alkyl complex

A rare uranium(III) alkyl complex, Tp*(2)U(CH(2)Ph) (2) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by salt metathesis from Tp*(2)UI (1) and KCH(2)Ph and fully characterized using (1)H NMR, infrared, and electronic absorption spectroscopies as well as X-ray crystallography. This complex has a uranium-carbon distance of 2.57(2) ?, which is comparable to other uranium alkyls reported. Treating this compound with either carbon dioxide or carbon disulfide results in insertion into the uranium-carbon bond to generate Tp*(2)U(κ(2)-O(2)CCH(2)Ph) (3) and Tp*(2)U(SC(S)CH(2)Ph) (4), respectively. These species, characterized spectroscopically and by X-ray crystallography, feature new carboxylate and dithiocarboxylate ligands. Analysis by electronic absorption spectroscopy supports the trivalent oxidation state of the uranium center in both of these derivatives. Addition of trimethylsilylhalides (Me(3)SiX; X = Cl, I) to 3 results in the release of the free silyl ester, Me(3)SiOC(O)CH(2)Ph, forming the initial uranium monohalide species, Tp*(2)UX, which can then be used over multiple cycles for the functionalization of carbon dioxide.     

Pyrazolylborate-zinc-nucleobase-complexes, 2:(1) preparations and structures of Tp(Cum,Me)Zn and Tp(Ph,Me)Zn complexes

The interactions of the nine most significant nucleobases (thymine, uracil, dihydrouracil, cytosine, adenine, guanine, diaminopurine, xanthine, hypoxanthine, in their deprotonated forms) with zinc and with themselves in pyrazolylborate zinc complexes Tp(Cum,Me)Zn-base and Tp(Ph,Me)Zn-base are described. Except for guanine, the complexes Tp*Zn-base could be isolated in all cases. Structure determinations could be performed for seven of the eight product types. Except for dihydrouracil and xanthine, the zinc ion is attached to that nitrogen of the base which in nucleosides bears the sugar moiety. In the solid state, all zinc-bound nucleobases are involved in hydrogen bonding interactions. Except for xanthine, this includes homo base pairing across a crystallographic inversion center.     

Synthesis and structural comparison of a series of divalent Ln(Tp(R,R)')2 (Ln = Sm, Eu, Yb) and trivalent Sm(Tp(Me2))2X (X = F, Cl, I, BPh4) complexes

Reaction of LnI2 (Ln = Sm, Yb) with two equivalents of NaTp(Me2) or reduction of Eu(Tp(Me2))2OTf gives good yields of the highly insoluble homoleptic Ln(II) complexes, Ln(Tp(Me2))2 (Ln = Sm (1a), Yb (2a), Eu (3a)). Use of the additionally 4-ethyl substituted Tp(Me2,4Et) ligand produces the analogous, but soluble Ln(Tp(Me2,4Et))2 (1-3b) complexes. Soluble compounds are also obtained with the Tp(Ph) and Tp(Tn) ligands (Tn = thienyl), Ln(Tp(Ph))2 (Ln = Sm, 1c; Yb, 2c) and Ln(Tp(Tn))2 (Ln = Sm, 1d; Yb, 2d). To provide benchmark parameters for structural comparison the series of Sm(Tp(Me2))2X complexes (X = F, 1e; Cl, 1f; Br, 1g; I, 1h; BPh4, 1j) were prepared either via oxidation of the Sm(Tp(Me2))2 or salt metathesis from SmX3 (X = Cl, Br, I). The solid-state structures of 1-3a, 1b, 1-2c and 1e, 1f, 1h, and 1j were determined by single-crystal X-ray diffraction. The homoleptic bis-Tp complexes are all six-coordinate with trigonal antiprismatic geometries, planes of the kappa(3)-Tp ligands are parallel to one another. In the series of Sm(Tp(Me2))2X complexes the structure changes from seven-coordinate molecular compounds, with intact Sm-X bonds, for X = F, Cl, to six-coordinate ionic structures [Sm(Tp(Me2))2]X (X = I, BPh4), suitable crystals of the bromide compound could not be obtained. The dependence of the structures on the size of X is understandable in terms of the interplay between the size of the cleft that the [Sm(Tp(Me2))2](+) fragment can make available and the donor ability of the anionic group toward the hard Sm(III) center.     

Chemistry of alkoxo complexes,part II. Synthesis and characterization of some mono(chelato)mono(alkoxo)nickel(II) complexes

Summary Mono(chelato)mono(alkoxo)nickel(II) complexes of the general formula, Ni(OR)(dk) (R = Me, i-Pr and dk = acac, bazac, dbzm and tta) have been synthesized by the reactions of nickel alkoxides, Ni(OR)₂(R = Me and i-Pr) with acetylacetone (Hacac), benzoylacetone (Hbzac), dibenzoylmethane (Hdbzm) and 2-thienoyltrifluoroacetone (Htta). These derivatives interchange their alkoxy groups quantitatively with other alcohols. Molecular weight, i.r., electron spin resonance, electronic reflectance spectral and magnetic susceptibility studies have been carried out.Part. 1.Transition Met. Chem.,2, 204 (1977)     

Solid-state (15)N CPMAS NMR and computational analysis of ligand hapticity in rhodium(η-diene) poly(pyrazolyl)borate complexes

Novel [Rh(η-diene)Tp(x)] complexes of sterically encumbered Tp(x) ligands (Tp(x) = Tp(4Bo), diene = cod, 1; nbd, 2; Tp(x) = Tp(4Bo,5Me), diene = cod, 3; nbd, 4; Tp(x) = Tp(a,3Me), diene = cod, 5; nbd, 6; Tp(x) = Tp(a*,3Me), diene = cod, 7; nbd, 8) have been prepared by treatment of [Rh(η-diene)(μ-Cl)](2) with TlTp(x) (Tp(x) in general, in detail: Tp(4Bo) = hydrotris(indazol-1-yl)borate, Tp(4Bo,5Me) = hydrotris(5-methyl-indazol-1-yl)borate, Tp(a,3Me) = hydrotris(3-methyl-2H-benz[g]-4,5-dihydroindazol-2-y1)borate, Tp(a*,3Me) = hydrotris(3-methyl-2H-benz[g]indazol-2-yl)borate), and characterized by analytical and spectral data (IR, (1)H, (11)B, and (13)C NMR solution). The structures adopted by [Rh(nbd)Tp(4Bo)] 2, [Rh(cod)Tp(4Bo,5Me)] 3, [Rh(nbd)Tp(a,3Me)] 6, [Rh(nbd)Tp(a*,3Me)] 8, and [Rh(nbd)Tp(a*,3Me*)] 8* (incorporating a borotropomeric ligand), have been investigated. Low steric hindrance between the ligands in 2 and 3 permits κ(3) coordination of the pyrazolylborate while the high steric encumbrance present in 6, 8, and 8* results in κ(2) ligands. The coordination modes of the ligands to the metal have also been established by (15)N CPMAS studies of selected ligands and their corresponding Rh complexes. These spectroscopic data are in agreement with the (15)N chemical shifts obtained by using quantum-chemical methods to assist reliable assignments of the experimental values, affording new insights into the extraction of structural information concerning the hapticity (κ(2) or κ(3)) of the poly(pyrazolyl)borate ligands to the Rh metal.     

Neutral and cationic aluminium complexes of a sterically demanding N-imidoylamidine ligand

The N-imidoylamidine ligand i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2 2 was prepared. Direct reactions with AlI3 or AlMe3 afforded [(i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2)AlI2][AlI4] 3 and [i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2)AlMe2][AlMe4].AlMe3, 4 respectively. Thermolysis of 4 gave (i-Pr2C6H3NC(=CH2)(NC6H3i-Pr2)(C(Me)NC6H3i-Pr2)AlMe2 6. Subsequent reaction with B(C6F5)3 gave the zwitterionic species [(i-Pr2C6H3)N(C(=CH2)NC6H3i-Pr2)(C(Me)NC6H3i-Pr2)AlMe(mu-MeB(C6F5)3)] 7. In a related reactions of 2, [Ph3C][B(C6F5)4] and AlMe3, AlH3.NEtMe2 or AlD3.NMe3, the complexes [(i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2)AlR2][B(C6F5)4] (R = Me 5, H 8, D 9) and [(i-Pr2C6H3)N(C(=CH2)NC6H3i-Pr2)(C(Me)NC6H3i-Pr2)AlH][B(C6F5)4] 10 are formed. Single-crystal X-ray data for 2, 3, 5 and 10 are reported.     

Theory of late-transition-metal alkyl and heteroatom bonding: analysis of Pt, Ru, Ir, and Rh complexes

Density functional and correlated ab initio methods were used to calculate, compare, and analyze bonding interactions in late-transition-metal alkyl and heteroatom complexes (M-X). The complexes studied include: (DMPE)Pt(CH(3))(X) (DMPE = 1,2-bis(dimethylphosphino)ethane), Cp*Ru(PMe(3))(2)(X) (Cp* = pentamethylcyclopentadienyl), (DMPE)(2)Ru(H)(X), (Tp)(CO)Ru(Py)(X) (Tp = trispyrazolylborate), (PMe(3))(2)Rh(C(2)H(4))(X), and cis-(acac)(2)Ir(Py)(X) (acac = acetylacetonate). Seventeen X ligands were analyzed that include alkyl (CR(3)), amido (NR(2)), alkoxo (OR), and fluoride. Energy decomposition analysis of these M-X bonds revealed that orbital charge transfer stabilization provides a straightforward model for trends in bonding along the alkyl to heteroatom ligand series (X = CH(3), NH(2), OH, F). Pauli repulsion (exchange repulsion), which includes contributions from closed-shell d(π)-p(π) repulsion, generally decreases along the alkyl to heteroatom ligand series but depends on the exact M-X complexes. It was also revealed that stabilizing electrostatic interactions generally decrease along this ligand series. Correlation between M-X and H-X bond dissociation energies is good with R(2) values between 0.7 and 0.9. This correlation exists because for both M-X and H-X bonds the orbital stabilization energies are a function of the orbital electronegativity of the X group. The greater than 1 slope when correlating M-X and H-X bond dissociation energies was traced back to differences in Pauli repulsion and electrostatic stabilization.     

Arylthiolate coordination and reactivity at pseudotetrahedral nickel(II) centers: modulation by noncovalent interactions

Five new pseudotetrahedral nickel(II) arylthiolate complexes Tp (R,Me)Ni-SR' [(Tp (R,Me)) (-) = 2,2,2-kappa (3)-hydridotris(3-R,5-methylpyrazolyl)borate; R = Me, R' = C 6H 5 (Ph), 2,4,6-C 6H 2(CH 3) 3 (Mes); R = Ph, R' = C 6H 5 (Ph), 2,4,6-C 6H 2(CH 3) 3 (Mes), and 2,6-C 6H 3(CH 3) 2 (Xyl)] were prepared by metathesis reactions of known chloride complexes with sodium arylthiolate salts in THF. The new products were fully characterized. The effect of increasing bulk of substituents at the proximal 3-pyrazolyl and ortho-thiolate positions represented in this series was evident in spectroscopic studies (UV-vis-NIR, (1)H NMR) of the product complexes. Increased steric contact induced red-shifting of nickel-thiolate ligand to metal charge transfer (LMCT) bands and enhanced contact shifts of arylthiolate protons with the paramagnetic ( S = 1) nickel(II) ion. These spectroscopic effects arise from structural distortion of the nickel(II)-thiolate bond revealed by X-ray crystal structure determinations of the structural extremes of the series, Tp (Me,Me)Ni-SPh and Tp (Ph,Me)Ni-SXyl. The distortion consists of a significantly increased tilting of the Ni-S bond from an ideal trigonal axis and increased linearity of the Ni-S-R angle that alters covalency of the Ni-S coordinate bond. Reactivity of the nickel-thiolate linkage toward electrophilic alkylation with MeI is also significantly affected, showing enhanced rates according to two distinct competing mechanisms, direct bimolecular alkylation of intact complex and rate-limiting unimolecular dissociation of free thiolate. Possible biochemical relevance of these observations to tetrahedral nickel(II) centers in metalloenzymes is considered.     

Zinc(II)/ketoxime system as a simple and efficient catalyst for hydrolysis of organonitriles

The hydrolysis of sterically hindered and unhindered alkyl nitriles, and also of benzyl and phenyl nitriles RCN (R = Me, CH(2)Cl, Et, n-Pr, i-Pr, n-Bu, t-Bu, p-MeOC(6)H(4)CH(2), Ph), to carboxamides is catalyzed by a novel system of superior simplicity consisting of cheap, widely commercially available, and rather environmentally friendly compounds, that is, a ZnX(2)/ketoxime combination, but it does not proceed at all with either the zinc salt or the ketoxime taken alone. The nature of the anion X(-) in the zinc salt (X = NO(3), Cl, CF(3)SO(3)) or of the ketoxime (Me(2)C=NOH, C(4)H(8)C=NOH, C(5)H(10)C=NOH) does not affect strongly the catalytic properties of the system, but the best results were obtained so far with a Zn(NO(3))(2).6H(2)O/2-propanone oxime molar ratio of 1:4; turnover numbers are typically above ca. 100 but reach as high as 1000 for p-MeOC(6)H(4)CH(2)C(=O)NH(2). The previously unknown structures of the two carboxamide products n-BuC(=O)NH(2) and p-MeOC(6)H(4)CH(2)C(=O)NH(2) were determined by X-ray diffraction studies. The complexes [ZnX(2)(R(2)C=NOH)(2)] (X = Cl, R(2) = 2Me, C(4)H(8), C(5)H(10); X = NO(3), R = C(4)H(8)), prepared by heating the appropriate zinc salts with 2 equiv of the ketoxime in acetone and characterized by C, H, N analyses, FAB-MS, (1)H and (13)C[(1)H] NMR spectroscopies, and also X-ray crystallography (for X = Cl, R(2) = 2Me; X = NO(3), R = C(4)H(8)), proved to be catalyst precursors in the conversions because the activity of these species is high only in the presence of 2 equiv of the ketoxime.     

Ni(II) complexes featuring non-metallated pincer-type ligands

Attempts to prepare pincer-type Ni complexes from the ligands (i-Pr(2)POCH(2))(2)CH(2) and (pz*CH(2))(2)CH(2) (pz* = 3,5-dimethylpyrazol-1-yl) gave instead the complexes cis-{kappa(P),kappa(P')-(i-Pr(2)POCH(2))(2)CH(2)}NiCl(2) and {kappa(N),kappa(N')-(pz*CH(2))(2)CH(2)}NiBr(2). X-Ray diffraction studies confirmed that these potentially pincer-type ligands have not undergone metallation, serving instead as chelating ligands in essentially square-planar or tetrahedral complexes. Heating of these compounds failed to induce metallation of the coordinated ligands.     

Synthesis and characterization of mixed-substituent N-silylphosphoranimines

The preparation of a large series of new N-silyl-P-alkylphosphoranimines and their (silylamino)phosphine precursors is reported. Oxidative bromination of the P-functional (silylamino)phosphines, (Me(3)Si)(2)NP(R)X [R = n-Pr, n-Bu, i-Pr, t-Bu; X = Br, OR' (R' = CH(2)CF(3), Ph)], occurred smoothly at 0 degrees C and afforded the desired P-bromophosphoranimines, Me(3)SiN=P(R)(X)Br. Nucleophilic substitution reactions of the P-dibromo members of this series with LiOR' gave the corresponding P-trifluoroethoxy- and P-phenoxyphosphoranimines, Me(3)SiN=P(R)(OR')(2) (R' = CH(2)CF(3), Ph). All of these N-silylphosphoranimines, which are potential precursors to new cyclic and/or polymeric phosphazenes, were obtained as thermally stable, distillable liquids and were characterized by NMR ((1)H, (13)C, and (31)P) spectroscopy and elemental analysis.     

Ligand-modification effects on the reactivity, solubility, and stability of organometallic tantalum complexes in water

Tantalum complexes [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NMe(2))=CH)py}] (4) and [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NH(2))=CH)py}] (5), which contain modified alkoxide pincer ligands, were synthesized from the reactions of [TaCp*Me{κ(3)-N,O,O-(OCH(2))(OCH)py}] (Cp* = η(5)-C(5)Me(5)) with HC≡CCH(2)NMe(2) and HC≡CCH(2)NH(2), respectively. The reactions of [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(Ph)=CH)py}] (2) and [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(SiMe(3))=CH)py}] (3) with triflic acid (1:2 molar ratio) rendered the corresponding bis-triflate derivatives [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(OCHC(Ph)=CH(2))py}] (6) and [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(OCHC(SiMe(3))=CH(2))py}] (7), respectively. Complex 4 reacted with triflic acid in a 1:2 molar ratio to selectively yield the water-soluble cationic complex [TaCp*(OTf){κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NHMe(2))=CH)py}]OTf (8). Compound 8 reacted with water to afford the hydrolyzed complex [TaCp*(OH)(H(2)O){κ(3)-N,O,O-(OCH(2))(OCHC(CH(2)NHMe(2))=CH(2))py}](OTf)(2) (9). Protonation of compound 8 with triflic acid gave the new tantalum compound [TaCp*(OTf){κ(4)-C,N,O,O-(OCH(2))(HOCHC(CH(2)NHMe(2))=CH)py}](OTf)(2) (10), which afforded the corresponding protonolysis derivative [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(HOCHC(CH(2)NHMe(2))=CH(2))py}](OTf) (11) in solution. Complex 8 reacted with CNtBu and potassium 2-isocyanoacetate to give the corresponding iminoacyl derivatives 12 and 13, respectively. The molecular structures of complexes 5, 7, and 10 were established by single-crystal X-ray diffraction studies.