Nucleophilic attack on η³-allyl and η²-tetrahydroborate complexes of ruthenium(ii)

Contrasting Photochemical and Thermal Catalysis by Ruthenium Arsine Complexes Revealed by Parahydrogen Enhanced NMR Spectroscopy

“…Magnetization transfer from 9 a into free cis ‐stilbene was observed in the corresponding EXSY spectra at 358 K thereby confirming that this complex plays a direct role in alkyne hydrogenation (Figure 3b) in a process that presumably requires CO loss. In support of this, we note the related complexes Ru(R)(H)(CO) 2 (L) 2 (where L=PMe 2 Ph or PMe 3 and R=Ph or Et) slowly eliminate ethane and benzene at 295 K [78–80] …”

Use of the tetrahydroborate ligand as “gate-keeper” and protected hydride ligand: preparation and study of alkyl hydride and acyl hydride complexes of ruthenium(ii)

“…It is known, however, that the rate-determining step in the conversion of methyl complexes of ruthenium(II) [Ru(CO) 2 (Me)X(PMe 2 Ph) 2 ] (X = Me or Ph) to acyl complexes [Ru(CO)(COMe)X(PMe 2 Ph) 2 L ] does not involve the incoming ligand L , 35 and it seems likely that the same applies to the opening of an Ru • • • HB bridge, particularly since the 1 H spectrum of 3 shows that bridge opening (and reclosing) occurs rapidly even in the absence of a nucleophile and in a non-nucleophilic solvent such as CD 3 C 6 D 5 . 23 On this basis, the pathway from 3 to complexes 5a-d should be the same for all four complexes, which means that the rate of conversion of the intermediates 4b-d to 5b-d must be appreciably faster than that for the conversion of 4a to 5a. On steric grounds, one would anticipate that combination of ethyl and carbonyl ligands would be more rapid for 4a than for 4b and 4d, since PMe 2 Ph has a greater cone angle 50 than either P(OMe) 3 or CO.…”

“…The value of the ability of the tetrahydroborate ligand to act as a protected hydride ligand is illustrated by the ease with which the readily prepared [Ru(g 2 -BH 4 )(CO)H(PMe 2 Ph) 2 ], 2, can be converted first to the ethyl complex 3 23 and then to complexes of the type 4 or 5, before the protection is removed under conditions which are still mild enough to allow study of the resulting alkyl hydride complexes [Ru(CO)(Et)H(PMe 2 Ph) 2 L ] {1a, L = PMe 2 Ph; 1b, L = P(OMe) 3 } and acyl hydride complexes [Ru(COEt)H(PMe 2 Ph) 2 L 2 ] {8c, L = Me 3 CNC; 8d, L = CO}. Which type of complex is obtained presumably depends on the relative stabilities of the precursors 4 and 5: as discussed above, the greater the p-accepting ability of the ligand L is, the less serious should be the loss of much of the p-acceptor character of the carbonyl ligand in 4 as it becomes part of the acyl ligand in 5.…”

“…We had now demonstrated that it was possible, under extremely mild conditions, to obtain propanal by first treating [Ru(g 2 -BH 4 )(CO)H(PMe 2 Ph) 2 ], 2, with ethene, 23 and then treating the product of this reaction, 3, with CO. This sequence is not, however, catalytic, as the hydrogen required for the aldehyde is derived from 2, not from H 2 , and BH 3 is abstracted during the sequence.…”

“…Complex 1a can be conveniently obtained in two steps. First, the tetrahydroborate complex [Ru(g 2 -BH 4 )(CO)H(PMe 2 Ph) 2 ], 2, 23 is converted to [Ru(g 2 -BH 4 )(CO)Et(PMe 2 Ph) 2 ], 3, by treatment with ethene at or below 273 K. This reaction is of interest in its own right, since NMR studies showed 23 that it proceeds by way of the g 1 -tetrahydroborate complex [Ru(g 1 -BH 4 )(CO)(g 2 -C 2 H 4 )H(PMe 2 Ph) 2 ], making it a nice example of Marks and Kolb's gate-keeper effect. 15 Then 3 is treated with PMe 2 Ph, yielding 1a and H 3 B•PMe 2 Ph.…”