Determination of structural features of electrogenerated trans-[MoCl(NMe)(Ph2PCH2CH2PPh2)2]2+ by multinuclear electron paramagnetic resonance and electron nuclear double resonance spectroscopy and comparison of interatomic distances with those measured by X-ray analysis of the parent monocation
“…An ORTEP view is illustrated in Figure , and important bond distances and angles are given in Table . The Mo(1)−N(1) and N(1)−C(1) bond distances at 1.704(9) and 1.46(1) Å, respectively, and the linear Mo(1)−N(1)−C(1) linkage (178.3(8)°) confirm that the C(1) atom, which was originally a cyano carbon, has been doubly protonated to form the imido ligand. ,, On the other hand, the nitrile-enolato ligand shows structural characteristics similar to those observed for 5k ·C 6 H 6 . 3 Structure of the cationic part in trans -[Mo{NCH 2 CH 2 CO(C 6 H 4 Cl-4)}{NCCHCO(C 6 H 4 Cl-4)}(dppe) 2 ][OTf] ( 6c + [OTf] - ).…”
The molybdenum dinitrogen complex trans-[Mo(N2)2(dppe)2] (2) reacted with 2−2.5 equiv of various
β-ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans-[Mo(N)(NCCR1COR2)(dppe)2] (4; R1 = H, R2 = 4-MeOCOC6H4, 4-ClC6H4, 4-Tol, 4-MeOC6H4, 2-C4H3O, 2-C4H3S, Pr
i
; R1
= CN, R2 = Me, Ph; dppe = Ph2PCH2CH2PPh2) via the C⋮N triple bond cleavage of the nitriles on the
molybdenum center. On the other hand, the reaction of complex 2 with 2 equiv of pivaloylacetonitrile at room
temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans-[Mo(NCHCH2COBu
t
)(NCCHCOBu
t
)(dppe)2] (5k), which further underwent the cleavage of the CN double bond of the
alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans-[Mo(N)(NCCHCOBu
t
)(dppe)2] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of
4-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt3][OTf] (OTf = OSO2CF3) resulted in
the isolation of the cationic (imido)(nitrile-enolato) complex trans-[Mo{NCH2CH2CO(C6H4Cl-4)}{NCCHCO(C6H4Cl-4)}(dppe)2][OTf] (6c
+[OTf]-). The solid-state structures of 4h·1.5C2H4Cl2 (R1 = CN, R2 = Me),
5k·C6H6, and 6c
+[OTf]- were determined by single-crystal X-ray analyses. The detailed NMR analysis of the
reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans-[Mo(NCHCH2COR)(NCCHCOR)(dppe)2] (5) act as the key intermediates for the C⋮N triple bond fission, and
the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation
with the Hammett σ
p or σ
a constants for the aroyl substituents, where positive ρ values were obtained (ρ
p,
1.42; ρ
a, 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the
substitution of a dinitrogen ligand in 2 with a β-ketonitrile molecule, (2) the fast protonation of the nitrile
ligand by a second β-ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton
shift from the α-position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an
enolated imido ligand, and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido
complex 4 as the final product.
“…An ORTEP view is illustrated in Figure , and important bond distances and angles are given in Table . The Mo(1)−N(1) and N(1)−C(1) bond distances at 1.704(9) and 1.46(1) Å, respectively, and the linear Mo(1)−N(1)−C(1) linkage (178.3(8)°) confirm that the C(1) atom, which was originally a cyano carbon, has been doubly protonated to form the imido ligand. ,, On the other hand, the nitrile-enolato ligand shows structural characteristics similar to those observed for 5k ·C 6 H 6 . 3 Structure of the cationic part in trans -[Mo{NCH 2 CH 2 CO(C 6 H 4 Cl-4)}{NCCHCO(C 6 H 4 Cl-4)}(dppe) 2 ][OTf] ( 6c + [OTf] - ).…”
The molybdenum dinitrogen complex trans-[Mo(N2)2(dppe)2] (2) reacted with 2−2.5 equiv of various
β-ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans-[Mo(N)(NCCR1COR2)(dppe)2] (4; R1 = H, R2 = 4-MeOCOC6H4, 4-ClC6H4, 4-Tol, 4-MeOC6H4, 2-C4H3O, 2-C4H3S, Pr
i
; R1
= CN, R2 = Me, Ph; dppe = Ph2PCH2CH2PPh2) via the C⋮N triple bond cleavage of the nitriles on the
molybdenum center. On the other hand, the reaction of complex 2 with 2 equiv of pivaloylacetonitrile at room
temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans-[Mo(NCHCH2COBu
t
)(NCCHCOBu
t
)(dppe)2] (5k), which further underwent the cleavage of the CN double bond of the
alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans-[Mo(N)(NCCHCOBu
t
)(dppe)2] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of
4-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt3][OTf] (OTf = OSO2CF3) resulted in
the isolation of the cationic (imido)(nitrile-enolato) complex trans-[Mo{NCH2CH2CO(C6H4Cl-4)}{NCCHCO(C6H4Cl-4)}(dppe)2][OTf] (6c
+[OTf]-). The solid-state structures of 4h·1.5C2H4Cl2 (R1 = CN, R2 = Me),
5k·C6H6, and 6c
+[OTf]- were determined by single-crystal X-ray analyses. The detailed NMR analysis of the
reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans-[Mo(NCHCH2COR)(NCCHCOR)(dppe)2] (5) act as the key intermediates for the C⋮N triple bond fission, and
the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation
with the Hammett σ
p or σ
a constants for the aroyl substituents, where positive ρ values were obtained (ρ
p,
1.42; ρ
a, 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the
substitution of a dinitrogen ligand in 2 with a β-ketonitrile molecule, (2) the fast protonation of the nitrile
ligand by a second β-ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton
shift from the α-position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an
enolated imido ligand, and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido
complex 4 as the final product.
“…This in turn, except in cases of very large g value anisotropy, is dominated by the dipolar contribution, and the metal-proton distance is then given simply by a constant divided by the cube root of the maximum anisotropic coupling. Such procedures were recently used in studies of a molybdenum complex by Hughes et al (1990) and also, for example, by Mustafi and Makinen (1988).…”
Electron-nuclear double-resonance (ENDOR) spectra of protons coupled to molybdenum(V) in reduced xanthine oxidase samples have been recorded. Under appropriate conditions these protons may be studied without interference from protons coupled to reduced iron-sulfur centers. Spectra have been obtained for the molybdenum(V) species known as Rapid, Slow, Inhibited, and Desulfo Inhibited. Resonances corresponding to at least nine protons or sets of protons are observed for all four species, with coupling constants in the range 0.08-4 MHz. Most of these protons do not exchange when 2H2O is used as solvent. Additional protons giving couplings up to 40 MHz are also detected. These correspond to EPR-detectable protons studied in earlier work. The strongly coupled protons may be replaced by 2H, through appropriate use of 2H2O or of 2H-substituted substrates, with consequent disappearance of the 1H resonances. In most cases the corresponding 2H ENDOR features have also been observed. The nature of the various coupled protons is briefly discussed. Results permit specific conclusions to be drawn about the structures of the Inhibited and Desulfo Inhibited species. In particular, the data indicate that the aldehyde residue of the Inhibited species has been oxidized and that the four protons derived from the ethylene glycol molecule in the Desulfo Inhibited species are not all equivalent. Recent assignments [Edmondson, D.E., & D'Ardenne, S.C. (1989) Biochemistry 28, 5924-5930] of the weakly coupled protons in the latter species appear not to be soundly based. The possibility of obtaining more detailed structural information from the spectra is briefly considered.(ABSTRACT TRUNCATED AT 250 WORDS)
“…Alkylimido complexes are usually prepared by alkylation of nitrido complexes. , As reported earlier by us, a special route can be used if ethylimido complexes are to be synthesized (Scheme ) . Here, acetonitrile serves as a source for the ethylimido ligand, thus avoiding formation of mixed counterions associated with the alkylation routes.…”
Reduction and protonation of Mo(IV) imido complexes with diphosphine coligands constitutes the second part of the Chatt cycle for biomimetic reduction of N2 to ammonia. In order to obtain insights into the corresponding elementary reactions we synthesized the Mo(IV) ethylimido complex [Mo(CH3CN)(NEt)(depe)2](OTf)2 (2-MeCN) from the Mo(IV)-NNH2 precursor [Mo(NNH2)(OTf)(depe)2](OTf) (1). As shown by UV-vis and NMR spectroscopy, exchange of the acetonitrile ligand with one of the counterions in THF results in formation of the so far unknown complex [Mo(OTf)(NEt)(depe)2](OTf) (2-OTf). 2-MeCN and 2-OTf are studied by spectroscopy and X-ray crystallography in conjunction with DFT calculations. Furthermore, both complexes are investigated by cyclic voltammetry and spectroelectrochemistry. The complex 2-OTf undergoes a two-electron reduction in THF associated with loss of the trans ligand triflate. In contrast, 2-MeCN in acetonitrile is reduced to an unprecedented Mo(III) alkylnitrene complex [Mo(NEt)(CH3CN)(depe)2]OTf (5) which abstracts a proton from the parent Mo(IV) compound 2-MeCN, forming the Mo(III) ethylamido complex 5H and a Mo(II) azavinylidene complex 6. Compound 5 is also protonated to the Mo(III) ethylamido complex 5H in the presence of externally added acid and further reduced to the Mo(II) ethylamido complex 7. The results of this study provide further support to a central reaction paradigm of the Schrock and Chatt cycles: double reductions (and double protonations) lead to high-energy intermediates, and therefore, every single reduction has to be followed by a single protonation (and vice versa). Only in this way the biomimetic conversion of dinitrogen to ammonia proceeds on a minimum-energy pathway.
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