Studies into the Mechanism of CO-Induced N₂ Cleavage Promoted by an Ansa-Hafnocene Complex and C–C Bond Formation from an Observed Intermediate

Abstract

Carbonylation of the hafnocene dinitrogen complex, [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-(t)Bu)Hf](2)(μ(2), η(2), η(2)-N(2)), yields the corresponding hafnocene oxamidide compound, arising from N(2) cleavage with concomitant C-C and C-N bond formation. Monitoring the addition of 4 atm of CO by NMR spectroscopy allowed observation of an intermediate hafnocene complex with terminal and bridging isocyanates and a terminal carbonyl. (13)C labeling studies revealed that the carbonyl is the most substitutionally labile ligand in the intermediate and that N-C bond formation in the bridging isocyanate is reversible. No exchange was observed with the terminal isocyanate. Kinetic data established that the conversion of the intermediate to the hafnocene oxamidide was not appreciably inhibited by carbon monoxide and support a pathway involving rate-determining C-C coupling of the isocyanate ligands. Addition of methyl iodide to the intermediate hafnocene resulted in additional carbon-carbon bond formation arising from CO homologation following nitrogen methylation. Similar reactivity with (t)BuNCO was observed where C-C coupling occurred upon cycloaddition of the heterocumulene. By contrast, treatment of the intermediate hafnocene with CO(2) resulted in formation of a μ-oxo hafnocene with two terminal isocyanate ligands.

Recent developments in the homogeneous reduction of dinitrogen by molybdenum and iron

“…In early transition metal chemistry, stoichiometric N 2 functionalisation now includes a range of N-C bond formation reactions, particularly through discoveries by Fryzuk 49 and Chirik. 50,51,52,53,54,55,56 Thus, a current challenge is to make these N-C bond forming reactions catalytic, possibly by using middle (Mo) and late (Fe) transition metals like those described above. Because these reactions can construct value-added chemical compounds, there is significantly more promise for commercialization for fine chemicals than for ammonia synthesis.…”

Insights into the Cascade Reaction of CO and Heteroallenes Mediated by Dinitrogen Hafnocene Complexes: The Indirect Effect of Nitride's Nucleophilicity

“…[5] The formation of an itride intermediate in this type of reaction was also confirmed by kinetics studies reported by the same group. [6] Moreover,r eactionm echanisms were explored computationally,a gain highlighting the formation of this nitridei ntermediate. [7,8] Interestingly,C hirik and coworkers went one step further to includet he use of other intriguing smallm olecules, such as CO 2 ,i nstead of as econd consecutive addition of CO.…”

“…[7,8] In the latter studies, only at erminal coordination of the NCO À group to aH fc enter is considered computationally.T his might be due to the different ligand environment (ansa-type ligands), which is found to play akey role in this type of reactivity. [6,9] Notably, 4 displaysastrong nucleophilic character at the nitride ligand,a se videnced by the nature of its respectiveK S-HOMO and the naturalc harge of the nitrogen (Figure 3). In addition, the geometry corresponding to the [{(h Chem.E ur.J.2016, 22,4743-4747 www.chemeurj.org coordination mode was proposed in the literature, [9] based on ap yridine stabilized adduct, buti ts existence is strongly questioned by the DFT calculations reported herein.…”

Activation of Dinitrogen‐Derived Hafnium Nitrides for Nucleophilic NC Bond Formation with a Terminal Isocyanate

“…Inspired by reports by Sobota [2] and Fryzuk, [3] our laboratory has been exploring "ligand-induced N 2 cleavage", whereby a reducing transition metal is combined with an incoming ligand to supply the requisite six electrons to cleave the NN bond. [4] With strongly activated zirconocene [5] and hafnocene [6] dinitrogen complexes, this approach has proven to be a general method for nitrogen-carbon bond formation using carbon monoxide as the incoming ligand.…”

“…Monitoring the addition of one equivalent of CH 3 OTf to a C 6 (Figure 2) assigned as Figure 3 and established the identity of 4 as the dihafnocene ureate complex, [{(h 5 -C 5 H 2 -1,2,4-Me 3 ) 2 Hf} 2 (m 2 -OTf)(m 2 -NCONH)]. [22] The hydrogen atom on the ureate core was located in the difference map and there was no evidence for a methyl group from the CH 3 OTf addition.…”