Cyclohexadienyl Niobium Complexes and Arene Hydrogenation Catalysis

Abstract

Hydrogenolysis of R[P2N2]NbCH2SiMe3 (where R[P2N2] = RP(CH2SiMe2NSiMe2CH2)2PR; R = cyclohexyl, Cy, or phenyl, Ph) in benzene or toluene causes hydride addition to the aromatic solvent resulting in the formation of the π-bonded complexes R[P2N2]Nb(η5-C6H7) (R = Cy, 1; R = Ph, 2) and R[P2N2]Nb(η5-C7H9) (R = Cy, 3; R = Ph, 4) in benzene and toluene, respectively. Performing the hydrogenation at a higher pressure of 29 atm at room temperature causes the catalytic hydrogenation of benzene to cyclohexane as determined by NMR spectroscopy and GC−MS analysis. The hydrogenation of toluene to methylcyclohexane can also be performed, but the turnover frequency is considerably lower. Examination of the solid residues from the high-pressure hydrogenations indicates the formation of the π-bonded complexes 1 and 2. The addition of 29 atm of H2 to these cyclohexadienyl derivatives in benzene or toluene, however, shows no hydrogenation products, indicating these species are not catalytically active.

Low-Valent Tungsten Aryloxide Compounds with Pyridine Ancillary Ligation: Intramolecular Reduction of Heterocyclic and Aromatic Rings Including Formation of an η5-Cyclohexadienyl Group

“…Recently Fryzuk et al have reported cyclohexadienyl complexes of niobium amidophosphine ligands. 21 Tilley and Gavenonis have also demonstrated the formation of an η 5 -cyclohexadienyl ring by hydride transfer from tantalum to an ortho-aryl ring in an amido ligand. In this case reversible transfer to the meta-position of a [Ta-NHAr*] ring (Ar* ) 2,6-dimesitylphenyl) takes place.…”

“…The first key intermediate proposed in the mechanistic pathway (Scheme 4) is an η 5 -cyclohexadienyl formed by initial attack of a metal-hydride on an arene ring. 21 This has previously been missing in aryloxide chemistry. Recently Fryzuk et al have reported cyclohexadienyl complexes of niobium amidophosphine ligands.…”

Titanium‐Mediated Catalytic Hydrogenation of Monocyclic and Polycyclic Arenes

“…Compared to [D 6 ]benzene, neat [D 8 ]toluene is more rapidly hydrogenated with nearly twice the TON over the first 20 h (Table ; Figures S28 and S38, SI). This is surprising, as in the case of the macrocyclic phosphine amide R [P 2 N 2 ]Nb(CH 2 SiMe 3 ), the catalytic competency of the niobium complex drops precipitously from benzene to toluene . Rothwell's niobium arene hydrogenation catalysts also show a decrease in relative rate from benzene to toluene, though no other correlation between hydrogenation rates and sterics can be drawn from his system .…”

“…This is surprising, as in the case of the macrocyclic phosphinea mide R [P 2 N 2 ]Nb(CH 2 SiMe 3 ), the catalytic competency of the niobium complex drops precipitously from benzene to toluene. [7] Rothwell's niobium arene [b] [D 6 ]cyclohexane 1.0 0.05 2.0 [D 8 ]toluene [b] [D 8 ]methylcyclohexane1 .8 0.09 2.9 naphthalene tetralin 4.4 0.22 7.4 anthracene octahydroanthracene 4.6 0.23 6.5 styrene [c] ethylbenzene9 .0 0.45 13.9 [c] hydrogenation catalysts also show ad ecrease in relative rate from benzene to toluene, thoughn oo ther correlation between hydrogenation rates and sterics can be drawn from his system. [1b] However, it should be noted that, when using fewer equivalents of toluene (20 equiv) in [D 12 ]cyclohexane with 1, the decrease in the rate of hydrogenation is pronounced (TON < 0.7 after 20 h) ( Figure S33, Supporting Information).…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…101 Silyl derivatives of group 5 organometallic complexes have been investigated including intramolecular C-H activation of silyl ligands by tantalum() imido moieties 102 and the formation of an unusual imido-bridged 1,3-disilaniocyclobutane 11 from reaction between [Nb(η 5 -C 5 H 5 )(H)(SiMe 2 I) 2 ] and H 2 NEt 2 . 103 Hydrogenation of aromatics 104 and intramolecular hydride transfers to aromatic rings have been studied and an investigation into dihydrogen addition to [Ta(CH 2 )-(η 5 -C 5 Me 5 ) 2 (H)] using parahydrogen has also been reported. 105 Insertion processes of a series of niobium() alkyl and alkyne complexes have been investigated and supported by DFT calculations 106 and the coupling of 1-aza-1,3-butadiene and iminoacyl ligands to give amido-imido complexes of tantalum() reported.…”