Understanding Terminal versus Bridging End-on N₂ Coordination in Transition Metal Complexes

Abstract

Terminal and bridging end-on coordination of N 2 to transition metal complexes offer possibilities for distinct pathways in ammonia synthesis and N 2 functionalization. Here we elucidate the fundamental factors controlling the two binding modes and determining which is favored for a given metal−ligand system, using both quantitative density functional theory (DFT) and qualitative molecular orbital (MO) analyses. The Gibbs free energy for converting two terminal MN 2 complexes into a bridging MNNM complex and a free N 2 molecule (2ΔG eq °) is examined through systematic variations of the metal and ligands; values of ΔG eq °range between +9.1 and −24.0 kcal/mol per M−N 2 bond. We propose a model that accounts for these broad variations by assigning a fixed π-bond order (BO π ) to the triatomic terminal MNN moiety that is equal to that of the free N 2 molecule, and a variable BO π to the bridging complexes based on the character (bonding or antibonding) and occupancy of the π-MOs in the tetratomic MNNM core. When the conversion from terminal to bridging coordination and free N 2 is associated with an increase in the number of π-bonds (ΔBO eq π > 0), the bridging mode is greatly favored; this condition is satisfied when each metal provides 1, 2, or 3 electrons to the π-MOs of the MNNM core. When each metal in the bridging complex provides 4 electrons to the MNNM π-MOs, ΔBO eq π = 0; the equilibrium in this case is approximately ergoneutral and the direction can be shifted by dispersion interactions.

Heterobimetallic-Mediated Dinitrogen Functionalization: N–C Bond Formation at Rhenium-Group 9 Diazenido Complexes

“…6,7,10 Bimetallic complexes are also capable of a "dissociative" activation strategy in which cleavage of N 2 to form two terminal nitrides occurs prior to N functionalization. 6,11,12 Recently, models have been developed to predict the tendency of metal complexes to form terminal monometallic versus bridging bimetallic dinitrogen complexes, 13 which in turn should aid the design of species with desired reactivity patterns. Finally, transition metal-mediated N 2 functionalization can also be facilitated through the use of noncovalent or hydrogen bonds, coordination of Lewis acids (LAs), or other secondary sphere interactions (similar to those in nitrogenase active sites) to enhance dinitrogen activation and reactivity.…”

“…Monometallic species are effectively limited to end-on coordination of N 2 , in which functionalization can occur either exclusively at the more nucleophilic distal nitrogen or in an alternating fashion at both N atoms. , In contrast, bimetallic species exhibit widely varied reactivity patterns due to the fact that electron density is often delocalized across the entire dinitrogen fragment; in addition, the N 2 ligand can be bound in either an end-on/end-on, side-on/side-on, or (the rare) end-on/side-on fashion (Scheme ). ,, Bimetallic complexes are also capable of a “dissociative” activation strategy in which cleavage of N 2 to form two terminal nitrides occurs prior to N functionalization. ,, Recently, models have been developed to predict the tendency of metal complexes to form terminal monometallic versus bridging bimetallic dinitrogen complexes, which in turn should aid the design of species with desired reactivity patterns. Finally, transition metal-mediated N 2 functionalization can also be facilitated through the use of noncovalent or hydrogen bonds, coordination of Lewis acids (LAs), or other secondary sphere interactions (similar to those in nitrogenase active sites) to enhance dinitrogen activation and reactivity. − For example, Szymczak and co-workers found that coordination of LAs to the distal nitrogen of iron dinitrogen complexes causes a “push–pull”-type activation of the dinitrogen fragment that primes these complexes for protonation at the distal N atom and disfavors undesired protonation at the Fe center …”

Mechanisms of Electrochemical N2 Splitting by a Molybdenum Pincer Complex

“…The features at 1595 and 1751 cm -1 clearly align with the Mo II species 5 t,t , while the feature at 1680 cm -1 is likely an isomer of 5 t,t , as the shift to more activated N2 stretches has been previously observed in Re2(µ-N2) complexes and is computational predicted upon isomerization of trans,trans M2(µ-N2) species. 38,39 Additionally, monitoring the chemical reduction of 2 with 1 equiv of CoCp*2 via rR revealed a minor feature at ca. 1680 cm -1 that decayed alongside concomitant growth of the feature at 1751 cm -1 that corresponds to 5 t,t (Figure S9).…”

(n-Bu)4NBr-Promoted N2 Splitting to Molybdenum Nitride

“…These linear N 2 -bridged complexes were proposed as key intermediates to give corresponding nitrides via zigzag transition states. Based on these experimental evidences, 10 π-electron configuration in the π-MO manifold has been recognized as a crucial parameter for the productive N 2 -splitting. ,, For thermally stable complexes L n MN 2 ML n , N–N cleavage initiated via light or protonation were also well explored. ,,− Especially, Masuda and co-workers reported a chemical or electrochemical one-electron oxidation of [Mo­(depe) 2 (N 2 ) 2 ] (depe = Et 2 PCH 2 CH 2 PEt 2 ) to give the Mo nitride product . Overall, except for highly reductive Mo­(III) supported by amide or silanolate ligands demonstrating unprecedented reactivity, most widespread strategies for N 2 splitting often depend on redox reaction conditions (Scheme A).…”

“…Based on these experimental evidences, 10 πelectron configuration in the π-MO manifold has been recognized as a crucial parameter for the productive N 2splitting. 1,16,17 For thermally stable complexes L n MN 2 ML n , N− N cleavage initiated via light or protonation were also well explored. 11,14,18−23 Especially, Masuda and co-workers reported a chemical or electrochemical one-electron oxidation of [Mo(depe) 2 (N 2 ) 2 ] (depe = Et 2 PCH 2 CH 2 PEt 2 ) to give the Mo nitride product.…”