ChemInform Abstract: Dynamics of Agostic Complexes of the Type C5R5(C2H4)M(CH2CH2‐μ‐H)+. Energy Differences Between and Ancillary Ligand Control of Agostic and Terminal Hydride Structures.

Abstract

046ChemInform Abstract The degenerate interconversions of the two enantiomeric forms of the agostic title complexes (M: Co; R: H, Me) via the sym. terminal hydrides (C5R5)Co(C2H4)2H+ are investigated by 13C NMR spectroscopy at 100.6 MHz in the temp. range -80 to -100 rc C. The determined free energies of activation for this process (5.3 kcal/mol) represent a maximum energy difference between the agostic and terminal hydride structures. The barrier for degenerate isomerization of the agostic Rh title complex (M: Rh; R: Me) via the terminal hydride is < 3.7 kcal/mol. Since complexes of the type (C5R5)(L)Rh(C2H4)H+ (L: PR3, P(OR)3) are terminal hydrides, these Rh systems represent a unique case where agostic and terminal hydride structures are sufficiently close in energy that the stable form observed can be altered by small change in the ancillary ligand.

[(dmpe)2MnH(C2H4)] as a Source of a Low-Coordinate Ethyl Manganese(I) Species: Reactions with Primary Silanes, H2, and Isonitriles

“…In contrast to reactions of trans-[(dmpe) 2 MnH(C 2 H 4 )] (1) with the secondary silanes Et 2 SiH 2 and Ph 2 SiH 2 (vide supra), 3 exposure of 1 to an excess of the primary silanes PhSiH 3 and n BuSiH 3 at 60 °C formed the disilyl hydride complexes [(dmpe) 2 MnH(SiH 2 R) 2 ] (4a, R = Ph; 4b, R = n Bu) with elimination of ethane (Scheme 2). 34 Both complexes gave rise to two 31 Crystals of seven-coordinate [(dmpe) 2 MnH(SiH 2 Ph) 2 ] (4a) were obtained by cooling a saturated solution in hexanes to −30 °C, and an X-ray structure (Figure 4) revealed an octahedral arrangement of the four phosphorus and two silicon atoms about manganese, with a disphenoidal arrangement of the phosphorus donors and the hydride ligand (located from the difference map) situated equidistant between the two silicon atoms. Complex 4a is the first structurally characterized disilyl hydride complex of manganese with nonchelating silyl ligands, although Tobita et al previously reported a series of disilyl hydride and silyl hydrosilane complexes in which both silicon-based donors are tethered by a xanthene backbone (Figure 5).…”

“…This reaction initially formed a low-symmetry species identified by NMR spectroscopy as the cis isomer of 6a, featuring 1 H NMR resonances for two diastereotopic MnCH 2 protons (−0.12 and 0.22 ppm; Figure 7) and three broad 31 P NMR signals (61.6, 74.4, and 81 ppm; broadening is presumably due to reversible isonitrile or phosphine donor dissociation in solution). Upon cooling to 207 K, the 31 P signals sharpened and one signal split into two, giving the expected four 31 P environments. At the temperature of synthesis (50 °C), cis-6a slowly converted into trans-6a (Scheme 3), which gave rise to a single sharp 31 P NMR signal at 74.7 ppm and an apparent octet in the 1 H NMR spectrum due to the MnCH 2 protons (0.47 ppm; apparent octet due to very similar 3 J H,H and 3 J H,P coupling to the adjacent CH 3 group and 4 equivalent phosphines; Figure 7).…”

“…Upon cooling to 207 K, the 31 P signals sharpened and one signal split into two, giving the expected four 31 P environments. At the temperature of synthesis (50 °C), cis-6a slowly converted into trans-6a (Scheme 3), which gave rise to a single sharp 31 P NMR signal at 74.7 ppm and an apparent octet in the 1 H NMR spectrum due to the MnCH 2 protons (0.47 ppm; apparent octet due to very similar 3 J H,H and 3 J H,P coupling to the adjacent CH 3 group and 4 equivalent phosphines; Figure 7). This cis−trans isomerization did not proceed to completion, but an equilibrium was established (over many days at 50 °C, or a few hours at 80 °C), dominated by trans-6a.…”

“…However, in several cases, there is experimental evidence to suggest that exchange between the hydride and alkene protons occurs via a β-agostic alkyl complex, with "in-place" exchange of the bridging hydrogen atom, rather than via a true low-coordinate (i.e., nonagostic) species. 14−18 This is indicated by dynamic NMR studies, including pairwise coalescence of the four 31 P NMR resonances observed for [(cis-Ph 2 PCHCHPPh 2 ) 2 MoH-(C 2 H 4 ) 2 ] + at low temperature, 15 exchange of the hydride and C 2 H 4 environments in [{κ 1 ,η 6 -Cy 2 P(o-C 6 H 4 )(o-C 6 H 4 NMe 2 )}-RuH(C 2 H 4 )] + without epimerization, 16 and exchange of the hydride and C 2 H 4 environments in [(κ 3 -POCOP)MH-(C 2 H 4 )] + (M = Rh, Ir; POCOP = o-C 6 H 3 (OP t Bu 2 ) 2 ) while top−bottom asymmetry is maintained. 17 In contrast to ethylene hydride complexes of second-and third-row transition metals, first-row transition metal examples are scarce (Figure 1), 2,19−23 and their isomerization to afford alkyl complexes has rarely been investigated.…”

“…In fact, for the complexes in Figure 1, an equilibrium with an ethyl isomer has only been reported for the cobalt complex (though in this case calculations suggested very similar energies for the nonagostic and β-agostic cobalt ethyl structures) 15,22 and the iron cyclopentadienyl complex [Cp*FeH(C 2 H 4 )(PMe 3 )]. 19 However, from the reverse perspective, the first-row transition metal (Sc, 24 Ti, 25,26 Ni, 27−29 and Co 30,31 ) ethyl complexes in Figure 2 all feature a β-agostic C−H−M interaction, and the cobalt, 30 nickel α-diimine, 29 and nickel β-diketiminate 28 complexes undergo NMR-observable exchange of the ethyl group CH 2 and CH 3 protons and/or carbon atoms, presumably via an undetected ethylene hydride complex.…”