Electronic Structure, Spectroscopic Properties, and Reactivity of Molybdenum and Tungsten Nitrido and Imido Complexes with Diphosphine Coligands:  Influence of the trans Ligand

Abstract

A series of molybdenum and tungsten nitrido, [M(N)(X)(diphos) 2 ], and imido complexes, [M(NH)(X)(diphos) 2 )]Y, (M ) Mo, W) with diphosphine coligands (diphos ) dppe/depe), various trans ligands (X ) N 3 -, Cl -, NCCH 3 ) and different counterions (Y -) Cl -, BPh 4 -) is investigated. These compounds are studied by infrared and Raman spectroscopies; they are also studied with isotope-substitution and optical-absorption, as well as emission, spectroscopies. In the nitrido complexes with trans-azido and -chloro coligands, the metal−N stretch is found at about 980 cm -1 ; upon protonation, it is lowered to about 920 cm -1 . The 1 A 1 f 1 E (n f π*) electronic transition is observed for [Mo(N)(N 3 )(depe) 2 ] at 398 nm and shows a progression in the metal−N stretch of 810 cm -1 . The corresponding 3 E f 1 A (π* f n) emission band is observed at 542 nm, exhibiting a progression in the metal−N stretch of 980 cm -1 . In the imido system [Mo(NH)(N 3 )(depe) 2 ]BPh 4 , the n f π* transition is shifted to lower energy (518 nm) and markedly decreases in intensity. In the trans-nitrile complex [Mo(N)(NCCH 3 )(dppe) 2 ]BPh 4 , the metal− N(nitrido) stretching frequency increases to 1016 cm -1 . The n f π* transition now is found at 450 nm, shifting to 525 nm upon protonation. Most importantly, the reduction of this nitrido trans-nitrile complex is drastically facilitated compared to its counterparts with anionic trans-ligands (E p red ) −1.5 V vs Fc + /Fc). On the other hand, the basicity of the nitrido group is decreased (pK a {[Mo(NH)(NCCH 3 )(dppe) 2 ](BPh 4 ) 2 } ) 5). The implications of these findings with respect to the Chatt cycle are discussed.

Energetics and Mechanism of Ammonia Synthesis through the Chatt Cycle: Conditions for a Catalytic Mode and Comparison with the Schrock Cycle

“…This is the counterpiece of our earlier theoretical investigations of the Schrock cycle, applying the same assumptions and quantum-chemical methodology (functional, basis sets, solvent correction etc.). Moreover, the present DFT investigation complements our spectroscopic and theoretical studies on important intermediates of the Chatt cycle, that is, Mo and W bis(dinitrogen complexes), diazenido(À) and hydrazido(2À) complexes, [18,23] hydrazidium complexes [24] and Mo nitrido/imido complexes, [21] all with diphosphine (dppe or depe) ligands. In view of the fact that the electronic structures, spectroscopic properties and chemical reactivities of these (and other) intermediates of the Chatt cycle have been defined and detailed insight into many elemental steps of the Chatt cycle has been achieved, [28] quantum chemistry can now be employed to establish an overall mechanistic picture of this chemistry and help to solve the remaining obstacles to a working catalytic system.…”

“…+ , [20] [ [18] e) [MoN(N 3 )A C H T U N G T R E N N U N G (dppe) 2 ], [21,22] [21] [MoBr(NH)A C H T U N G T R E N N U N G (dppe) 2 ]Br. [22] Bond length [] [24] Moreover, 4 a has a very low N-N-stretching frequency (236.5 cm…”

Chloridobis[1,2-bis(diphenylphosphino)ethane](ethylimido)molybdenum(IV) tetraphenylborate acetonitrile disolvate

“…For related literature, see: Alias et al (1997); Bruno et al (2002); Chatt & Dilworth (1977); George & Noble (1978); Mersmann et al (2006).…”

(Dinitrogen)molybdenum Complexes Supported by Asymmetric Silicon‐Centered Tripod Ligands: Steric and Electronic Influences on the Coordination of Mono‐ and Diphosphine Coligands

“…So far, a large number of N 2 complexes have been synthesized, only a few of which, however, allow the reduction of N 2 to NH 3 in a cyclic or even catalytic fashion. [11][12][13][14][15][16][17][18] More recently we focused our attention on the design of new ligand environments. On the basis of the latter systems, the first complete mechanistic scenario of N 2 reduction has been established by Chatt and co-workers.…”