Eric Wenger

Insertions of unsymmetric alkynes into the metal-carbon bonds of nickelacycles: What determines the regiochemistry?

Although the insertion of alkynes into transition-metal−carbon bonds plays an important role in synthesis, the regioselectivities observed with unsymmetric alkynes have usually been interpreted on the basis of steric effects. In this perspective paper, we review the available data for such reactions with nickelacycles and present the results of some preliminary theoretical (DFT) calculations. These suggest that, even for unactivated alkynes, the regiochemistry may also be controlled by electronic factors such as frontier-orbital interactions between the triple-bond of the alkyne and the polarized metal−carbon bond.

Insertion reactions of unsymmetrical ester-activated alkynes with o-benzylamine palladacycles: a regioselectivity study

Abstract The regioselectivities of the insertion reactions of RCCCO2Et (R=Ph, CF3) into the aryl–palladium bond of several five-membered, ortho-palladated dimethylbenzylamine complexes, [PdCl(C6H4CH2NMe2–κC,N)]2 (1), [PdCl(C6H4CH2NMe2–κC,N)(L)] [L=PEt3 (2), DMPP (3)], [Pd(C6H4CH2NMe2–κC,N)(NCMe)(L)]PF6 [L=MeCN (4), PEt3 (9), DMPP (5)], [Pd(OSO2CF3)(C6H4CH2NMe2–κC,N)(DMPP)] (6), [Pd(C6H4CH2NMe2–κC,N)(solvent)]PF6 (7) and [Pd(C6H4CH2NMe2–κC,N)(DMPP)(solvent)]PF6 (8), have been compared with the help of multinuclear NMR spectroscopy. In general, the carboxylate group in the resulting seven-membered palladacycles is preferentially located next to the phenyl group of the benzylamine moiety, but this substitution pattern can be reversed by use of complexes containing electron-deficient and/or coordinatively-unsaturated palladium centres. A mechanism, based both on the experimental results described in this paper and on DFT computations, is proposed.

Preparation of (ortho-halogeno)-η1-pyridylnickel(II) complexes, precursors for the generation of pyridyne-nickel(0) complexes☆

Abstract Monomeric η1-(3-chloro)-2-pyridyl complexes of nickel(II), NiCl(3-ClC5H3N-2) L2 [L2 = 2PEt3 (1), dcpe (2); dcpe = 1,2-bis(dicyclohexylphosphino)ethane, (C6H11)2 PCH2CH2P(C6H11)2], have been prepared by the reaction of 2,3-dichloropyridine with Ni(COD)2 in the presence of PEt3 or with Ni(η2-C2H4) (dcpe), respectively. Reaction of 2,3-dichloropyridine with NiCl2(PPh3)2 in the presence of zinc gave the dimeric (C,N)-bridged (3-chloro)-2-pyridyl complex [NiCl(μ-3-ClC5H3N-2)(PPh3)]2 (4), which re-forms 1 on addition of PEt3. A similar reaction between NiBr2(PPh3)2 and 3,4-dibromopyridine gave a mixture of the isomers NiBr(3-BrC5H3N-4)(PPh3)2 (5) and NiBr(4-BrC5H3N-3)(PPh3)2 (6), from which the PPh3 ligands could easily be replaced by dcpe to give NiBr(3-BrC5H3N-4)(dcpe) (7) and NiBr(4-BrC5H3N-3)(dcpe) (8). Alkali metal reduction of 2, and of the mixture of 7 and 8, gave unstable species that could not be isolated, which are thought to be nickel(0) complexes of 2,3- and 3,4-pyridyne, respectively, on the basis of their 31P NMR spectra. The structure of 4 has been solved by heavy atom methods and refined by least squares methods.

Simple method for the handling of crystals at low temperature: Application to the X-ray structure determination of Bu t 2 P(=NSiMe 3 )NHSiMe 3 and Bu t 2 P(NSiMe 3 ) 2

Crystals of the very soluble iminophosphonamide species But2P(=NSiMe3)NHSiMe3 (1) and [But2P(NSiMe3)2]Li·(thf)2 (2) have been obtained and were mounted at dry-ice temperature using a propan-2-ol matrix for X-ray data collection at 100 K. This very simple method for handling the crystals at low temperature is described together with the resulting molecular structures of (1) and (2).

Theoretical investigation of the formation of five-membered metallacycles by reaction of CO with metallacyclobutenones: differences between Ni and Pt

Abstract Reactions of CO with nickel or platinum complexes of cyclic alkynes or arynes form initially metallacyclobutenone species, which undergo further insertion of CO to give five-membered metallacycles. The differences in regioselectivity observed for nickel or platinum have been investigated computationally by DFT methods. Although both metals prefer insertion of the second CO via a five-coordinate mechanism, the formation of the five-membered nickelacycle is under thermodynamic control, leading to a symmetrical nickelacyclopentene-2,4-dione complex, whereas platinum yields the kinetically favoured platinacyclopentene-2,3-dione complex.

Successive insertion of tetrafluoroethylene and CO and of tetrafluoroethylene and acetylenes into aryne–nickel(0) bonds

Aryne–nickel complexes [Ni(η 2 -C 6 H 4 )L 2 ] [L 2 = 2PEt 3 or dcpe; dcpe = (C 6 H 11 ) 2 PCH 2 CH 2 P(C 6 H 11 ) 2 ] and [Ni(η 2 -C 10 H 6 )(PEt 3 ) 2 ] reacted readily with C 2 F 4 to form the corresponding five-membered tetrafluoro-substituted nickelacycles [Ni(C 6 H 4 CF 2 CF 2 -2)L 2 ] (L 2 = dcpe or 2PEt 3 ) and [Ni(2-C 10 H 6 CF 2 CF 2 -3)(PEt 3 ) 2 ], respectively. The complex [Ni(C 6 H 4 CF 2 CF 2 -2)(dcpe)] is very stable towards air, whereas the PEt 3 analogues react readily to give µ-aryloxo dimers. The naphthalene-based dimer [{Ni(µ-2-OC 10 H 6 CF 2 CF 2 -3)(PEt 3 )} 2 ] has been structurally characterized. The complexes [Ni(C 6 H 4 CF 2 CF 2 -2)L 2 ] insert CO into their aryl–nickel bonds to form six-membered acyl complexes [Ni{C(O)C 6 H 4 CF 2 C F 2 -2}L 2 ] (L 2 = dcpe or 2PEt 3 ) and, after CO-induced reductive elimination, 2,2,3,3-tetrafluoroindanone. The dcpe acyl complex has also been shown to undergo reaction with air to form the carboxylato complex [Ni{OC(O)C 6 H 4 CF 2 C F 2 -2}(dcpe)], whose structure has been confirmed by X-ray crystallography. Some insertions of acetylenes into the aryl–nickel bonds of [Ni(C 6 H 4 CF 2 CF 2 -2)L 2 ] are also reported.

Towards rational syntheses of the elusive metallocarbohedrenes: density functional prescriptions for electronic and geometric structures

The metallocarbohedrenes are binary molecular clusters containing metal atoms linked by acetylenediide C(2) groups. Hundreds of these molecules have been generated, detected and reacted in the gas phase since the prototype, [Ti(8)(C(2))(6)], was reported in 1992, but none has yet been synthesised pure in bulk: the time gap between detection and preparation increasingly exceeds that of the fullerenes. We report here the results of density functional calculations of geometrical and electronic structure of more than 150 postulated metallocarbohedrenes, stabilised by terminal ligation, in order to recognise the more electronically favourable and less reactive targets. At least 38 metallocarbohedrenes have been identified as having a spin singlet ground state, with a relatively large (> 0.5 eV) energy gap between HOMO and LUMO, and an appropriate HOMO energy. In addition, a considerable number of electronically stable metallocarbohedrenes are predicted to have highly paramagnetic ground states, potentially useful in molecular magnetism. The geometrical principles for enclosing but unstrained coordination of metal sites by terminal ligands are outlined. Mechanisms for rational syntheses are considered in the context of reaction type and precursor selection, including issues of oxidation and reduction, and kinetic versus thermodynamic control. This leads to many diverse reactions suggested for the rational syntheses of metallocarbohedrenes. Some preliminary experimental results are presented.

Preparation and reactivity of mononuclear platinum(0) complexes containing a η2-coordinated alkynylphosphine

Displacement of ethene from [Pt(η2-C2H4)(dcpe)] by Ph2PCCMe gives the monomeric η2-alkynylphosphine complex [Pt(η2-Ph2PCCMe)(dcpe)] (5). Reaction of 5 with one equivalent of HCl forms, in benzene, the four-coordinate η1-vinyl–platinum(II) complex [PtCl{C(CHMe)PPh2}(dcpe)] (6) by regiospecific addition of the proton to the alkyne carbon atom bearing the methyl group. In dichloromethane, reversible dissociation of the chloride ion from 6 takes place and the cation [Pt{C(CHMe)PPh2-κP,C1}(dcpe)]+ (6′) that contains a three-membered methylenephosphaplatinacycle fragment can be observed. Treatment of complex 5 with methyl iodide results in the formation of a phosphonium salt [Pt{η2-C(CH2)CHPPh2Me}(dcpe)]+I−, [8]I, in which the alkynylphosphine has rearranged to the corresponding allene. The alkynylphosphine complex 5 is cleanly oxidised at phosphorus in the presence of sulfur or air to give the corresponding sulfide or oxide [Pt(η2-Ph2P(X)CCMe)(dcpe)] (X = S (9), O (10)). Complexes 5, 6, 8, 9 and 10 have been structurally characterised by single-crystal X-ray diffraction analysis.

NEW DINUCLEAR PLATINUM(I) COMPLEX OBTAINED FROM THERMAL DEGRADATION OF PLATINUM(0)-TRIPHENYLPHOSPHINE COMPLEXES

Thermolysis of [Pt(PPh3)2(C2H4)], [Pt(PPh3)3] or of various complexes of the type [Pt(PPh3)2(alkyne)] in toluene gave the dinuclear platinum(I) complex [(Ph3P)Pt{µ-C6H4(PPh2)-2}(µ-PPh2)Pt(PPh3)] in ca. 60% yield as a consequence of both C–H and P–Ph cleavage of co-ordinated triphenylphosphine. The complex, which has been identified by mass spectrometry, NMR (31P, 195Pt) spectroscopy and single-crystal X-ray diffraction analysis, is probably identical with some of the compounds formulated in the literature either as [Pt2(PPh3)4] or [Pt2(µ-PPh2)2{C6H4(PPh2)-2}2]. The other product of the reaction is the known trinuclear compound [Pt3(µ-PPh2)3(Ph)(PPh3)2].

The seven-membered nickelacycle [NiBr{o-CH=C(CF3)C6H4CH2PPh2-κ2C,P}{PPh(CH2Ph)2}]

The crystal and molecular structures of the title compound, 3-bromo-3-(dibenzylphenylphosphonio)-2,2-diphenyl-5-trifluoromethyl-1H-benzo[e][1,2]phosphanickelepine, [NiBr(C(22)H(17)F(3)P)(C(20)H(19)P)], which was obtained as the major regioisomer from insertion of HCCCF(3) into the Ni-C bond of the five-membered phosphanickelacycle [NiBr(o-C(6)H(4)CH(2)PPh(2)-kappa2C,P)(PPh(CH(2)Ph)(2))], have been determined. Principal geometric data include the Ni-X bond lengths Ni-Br 2.3343 (4) A, Ni-P 2.1867 (7) and 2.2094 (7) A, and Ni-C 1.882 (3) A, and the two trans angles P-Ni-P 171.55 (3) degrees and Br-Ni-C 176.88 (9) degrees.