Jan-Meine Ernsting

Structural investigation of aryllithium clusters in solution I. A 13C and 7Li NMR study of phenyllithium and some methyl-substituted phenyllithium derivatives

Abstract 13C and 7Li NMR spectra of phenyllithium and several methyl subsituted phenyllithium derivatives have been recorded in the presence of known amounts of coordinating solvents such as monodentate diethyl ether and THF and the potentially bidentate TMEDA (tetramethylethylenediamine). The relative amounts of the tetrameric and dimeric aggregates identified in these spectra depend on the donor strength, on the amount and denticity of added donor, and the presence or absence of ortho substituents in the phenyl group. Discrete solvated tetrameric aggregates were formed upon addition of exactly one equivalent of the monodentate donor solvent to an aryllithium compound having no ortho substituents; the addition of either two equivalents or excess of monodentate donor solvent or one equivalent of bidentate donor ligand afforded dimeric species. When one or two methyl substituents were present ortho to the lithium-carbon bond, either a mixture of dimeric and tetrameric species was formed (one methyl group) or the dimeric species was exclusively formed (two methyl groups).

Structural investigations of aryllithium clusters in solution II. A. 13C and 7Li NMR study of substituted phenyllithium compounds containing ortho-directing NME2 and CH2NMe2 groups

Abstract 7Li and 13C NMR spectroscopic studies have been made on the behaviour of several aryllithium compounds containing a N,N-dimethylamino substituent suitably positioned for intramolecular coordination to lithium in the presence of accurately known quantities of coordinating solvents (diethyl ether, THF) and ligand (TMEDA). The solvent molecules compete with the internal nitrogen coordination, and if their donor strength is sufficient and the steric situation is favourable they can fully replace intramolecular N-coordination. In the presence of more than one equivalent of the solvent, the aryllithium cluster can be broken down to afford solvent coordinated dimeric species. This change is reflected in an upfield shift of ca. 10 ppm for the 13C NMR C-ipso signal, with a concomitant change of 1J(7Li,13C) from ca. 11 to 20 Hz, and by an upfield shift of the 7Li NMR signal by ca. 1 ppm.

Synthesis, characterization and reactivity of ionic palladium(II) complexes containing bidentate nitrogen ligands in a unidentate coordination mode,

Abstract The novel ionic complexes [Pd(Me)(p-An-BIAN)(LL)]SO3CF3 (LL=p-An-BIAN (bis(anisylimino)acenaphthene) (1a), phen (2a), dmphen (3a), dppe (4a), dppp (5a)) have been synthesized via the reaction of [Pd(Me)(NCMe)(p-An-BIAN)]SO3CF3 with LL. The X-ray crystal structure of complex 1a has been determined and shows a distorted square planar geometry in which one BIAN ligand is coordinated in a bidentate fashion (PdN(1)=2.037(4) A; PdN(2)=2.189(4) A) and, interestingly, the other BIAN ligand in a unidentate fashion (PdN(3)=2.066(4) A; PdN(4)=2.714(6) A). Spectroscopic data of the mixed ligand complexes [Pd(Me)(p-An-BIAN)(LL)]SO3CF3 (LL=phen (2a), dppe (4a), dppp (5a)) indicate that the LL ligand is coordinated in a bidentate fashion and the p-An-BIAN ligand in a unidentate fashion, which is in agreement with the larger complexation strength of the phen, dppe and dppp ligands, as compared with that of the p-An-BIAN ligand. In contrast, complex 3a (LL=dmphen) contains a bidentate p-An-BIAN ligand and a unidentate dmphen ligand, which can be explained by the sterically demanding methyl groups of the dmphen ligand. Complexes 1a–4a underwent insertion of carbon monoxide, resulting in the formation of acetylpalladium complexes [Pd(C(O)Me)(p-An-BIAN)(LL)]SO3CF3 (1b–4b). Since mass-law retardation by excess p-An-BIAN has been observed for CO insertion for complexes 1a and 4a, it is proposed that the mechanism involves dissociation of the unidentate nitrogen ligand. Complexes 1a–5a and 1b–4b show fluxional behavior due to flipping of the unidentate nitrogen ligand. Complexes 1a–3a and 1b–3b also show fluxional behavior due to site exchange of the nitrogen atoms of the bidentate nitrogen ligand. A mechanism for this exchange process has been proposed. This mechanism involves (a) substitution of a nitrogen atom of the bidentate nitrogen ligand by the uncoordinated nitrogen atom of the unidentate nitrogen ligand, (b) flipping of the unidentate nitrogen ligand, and (c) a second nitrogen substitution reaction. Reaction of the acetylpalladium complexes 1b–4b with norbornadiene led to dissociation of the unidentate nitrogen ligand and formation of the known alkylpalladium complexes [Pd(C7H8C(O)Me)(p-An-BIAN)]SO3CF3 (1c), [Pd(C7H8C(O)Me)(phen)]SO3CF3 (2c), 1c, and [Pd(C7H8C(O)Me)(dppe)]SO3CF3 (4c), respectively.

MULTINUCLEAR MAGNETIC RESONANCE STUDIES UNDER PRESSURE OF GASES AND IN SUPERCRITICAL MEDIA EMPLOYING A NOVEL TITANIUM-SAPPHIRE HIGH-PRESSURE CELL WITH PRESSURE SENSOR

A novel titanium–sapphire high‐pressure NMR cell equipped with a pressure sensor is presented. It enables the accurate and continuous in situ determination of the pressure in samples under pressure of gases and in supercritical CO2 . The homogeneity over the cell in commercial probes is excellent. The usefulness of this NMR cell is demonstrated in several 17O and 14N experiments in supercritical CO2. Furthermore, 1H NMR with concomitant monitoring of the hydrogen gas consumption during a hydrogenation reaction in solution is treated. Important characteristics are the accuracy combined with simplicity and flexibility of operation.

103Rh-NMR spectroscopy of some hydridorhodiumbis(carbonyl)diphosphine compounds

Abstract The 103 Rh chemical shifts of a series of hydridorhodiumbis(carbonyl)diphosphine compounds 1 – 17 and one phosphine–phosphite analogue 18 , containing chelating bidentate P-ligands, have been obtained by inverse HMQC detection sequences 1 H-{ 103 Rh} and 31 P-{ 103 Rh, 1 H}. These active hydroformylation catalysts are stable only under pressure of H 2 /CO (synthesis gas) and hence 1 H-, 31 P- and 103 Rh-NMR spectra have been recorded in a sapphire high-pressure NMR tube. The compounds HRh(CO) 2 (PP) exist as a mixture of equatorial–equatorial and equatorial–axial five-coordinate isomers for 1 – 17 , which are in a dynamic equilibrium that could not be brought into the slow exchange regime on the proton, the phosphorus or the rhodium NMR time scales. A correlation ( R =0.980) was found between δ ( 103 Rh) and the Hammett σ p -constant of the para -substituents Y of the P(C 6 H 4 Y- p ) 2 groups in the thixantphos ligand for the series of compounds 4 , 8 – 13 (lower δ ( 103 Rh) for electron withdrawing substituents). Correlation of δ ( 103 Rh) with Tolman basicity parameters (higher δ ( 103 Rh) with higher basicity) also gave a good fit ( R =0.955). The finding that correlations exist for this series between the ligand basicity, the ratio of equatorial–equatorial/equatorial–axial trigonal bipyramidal isomers and the δ ( 103 Rh) indicates that small structural and electronic changes in the ligands in the vicinity of transition metal nuclei have a small but significant influence on δ ( 103 Rh). This, together with other knowledge, may in the future serve to use Rh-NMR as an analytical tool in coordination chemistry.

Alkene rotation in [Ru(η5-C5H5) (L2) (η2-alkene][CF3SO3] with L2 = iPr- or pTol-diazabutadiene. X-ray crystal structure of [Ru(η5-C5H5(pTol-DAB) (η2-ethene][CF3SO3]

Reaction of RuCl(η5-C5H5(pTol-DAB) with AgOTf (OTf = CF3SO3) in CH2Cl2 or THF and subsequent addition of L′ (L′ = ethene (a), dimethyl fumarate (b), fumaronitrile (c) or CO (d) led to the ionic complexes [Ru(η5-C5H5)(pTol-DAB)(L′)][OTf] 2a, 2b and 2d and [Ru(η5-C5H5)(pTol-DAB)(fumarontrile-N)][OTf] 5c. With the use of resonance Raman spectroscopy, the intense absorption bands of the complexes have been assigned to MLCT transitions to the iPr-DAB ligand. The X-ray structure determination of [Ru(η5-C5H5)(pTol-DAB)(η2-ethene)][CF3SO3] (2a) has been carried out. Crystal data for 2a: monoclinic, space group P21/n with a = 10.840(1), b = 16.639(1), c = 14.463(2) A, β = 109.6(1)°, V = 2465.6(5) A3, Z = 4. Complex 2a has a piano stool structure, with the Cp ring η5-bonded, the pTol-DAB ligand σN, σN′ bonded (Ru-N distances 2.052(4) and 2.055(4) A), and the ethene η2-bonded to the ruthenium center (Ru-C distances 2.217(9) and 2.206(8) A). The C = C bond of the ethene is almost coplanar with the plane of the Cp ring, and the angle between the plane of the Cp ring and the double of the ethene is 1.8(0.2)°. The reaction of [RuCl(η5-C5H5)(PPh)3 with AgOTf and ligands L′ = a and d led to [Ru(η5-C5H5)(PPh3)2(L′)]OTf] (3a) and (3d), respectively. By variable temperature NMR spectroscopy the rottional barrier of ethene (a), dimethyl fumarate (b and fumaronitrile (c) in complexes [Ru(η5-C5H5)(L2)(η2-alkene][OTf] with L2 = iPr-DAB (a, 1b, 1c), pTol-DAB (2a, 2b) and L = PPh3 (3a) was determined. For 1a, 1b and 2b the barrier is 41.5±0.5, 62±1 and 59±1 kJ mol−1, respectively. The intermediate exchange could not be reached for 1c, and the ΔG# was estimated to be at least 61 kJ mol−. For 2a and 3a the slow exchange could not be reached. The rotational barrier for 2a was estimated to be 40 kJ mol−. The rotational barier for methyl propiolate (HC≡CC(O)OCH3) (k) in complex [Ru(η5-C5H5)(iPr-DAB) η2-HC≡CC(O)OCH3)][OTf] (1k) is 45.3±0.2 kJ mol−1. The collected data show that the barrier of rotational of the alkene in complexes 1a, 2a, 1b, 2b and 1c does not correlate with the strength of the metal-alkene interaction in the ground state.