Molybdenum complexes containing catecholate ligands. Structural studies on complexes of the pentaoxobis(quinone)dimolybdenate(n-) (n = 0, 1, 2) redox series
Abstract:Durch Oxidation von Mo(O) in Mo(CO)6 mit einer Reihe von Liganden in der o‐Benzochinonform erhält man wie früher mitgeteilt verschiedene Glieder der Reihe M02 O5 (chinon)? mit n: O, 1, 2. 9,10‐Phenanthrenchinon ergibt einen neutralen Komplex mit dem Liganden als Semichinon.
“…A number of complexes have been observed in which catecholate ligands bridge between multiple metal ions. [24][25][26][27][28][29][30] The majority of these compounds were isolated from organic solvents and employed the 3,5-di-tert-butylcatecholate or 3,4,5,6-tetrachlorocatecholate ligands. A limited number of complexes were prepared under aqueous conditions, however nonstandard preparation methods (e.g., precipitation followed by metathesis) were used.…”
Interactions between metals and catechol (1,2-dihydroxybenzene) or other ortho-dihydroxy moieties are being found in an increasing number of biological systems with functions ranging from metal ion internalization to biomaterial synthesis. Although metal-catecholate interactions have been studied in the past, we present the first systematic study of an array of these compounds, all prepared under identical conditions. We report the ultraviolet-visible absorption (UV-vis) spectra for catecholate and tironate complexes of the first row transition elements. Generation and identification of these species were accomplished by preparing aqueous solutions with varied ligand:metal ratios and subsequently titrating with base (NaOH). Controlled ligand deprotonation and metal binding resulted in sequential formation of complexes with one, two, and sometimes three catecholate or tironate ligands bound to a metal ion. We prepared the mono-, bis- and tris-catecholates and -tironates of Fe(3+), V(3+), V(4+)and Mn(3+), the mono- and bis-catecholates and -tironates of Cu(2+), Co(2+), Ni(2+), Zn(2+), Cr(2+) and Mn(2+), and several Ti(4+) and Cr(3+) species. The UV-vis spectra of each complex are described, some of which have not been reported previously. These data can now be applied to characterization of biological metal-catecholate systems.
“…A number of complexes have been observed in which catecholate ligands bridge between multiple metal ions. [24][25][26][27][28][29][30] The majority of these compounds were isolated from organic solvents and employed the 3,5-di-tert-butylcatecholate or 3,4,5,6-tetrachlorocatecholate ligands. A limited number of complexes were prepared under aqueous conditions, however nonstandard preparation methods (e.g., precipitation followed by metathesis) were used.…”
Interactions between metals and catechol (1,2-dihydroxybenzene) or other ortho-dihydroxy moieties are being found in an increasing number of biological systems with functions ranging from metal ion internalization to biomaterial synthesis. Although metal-catecholate interactions have been studied in the past, we present the first systematic study of an array of these compounds, all prepared under identical conditions. We report the ultraviolet-visible absorption (UV-vis) spectra for catecholate and tironate complexes of the first row transition elements. Generation and identification of these species were accomplished by preparing aqueous solutions with varied ligand:metal ratios and subsequently titrating with base (NaOH). Controlled ligand deprotonation and metal binding resulted in sequential formation of complexes with one, two, and sometimes three catecholate or tironate ligands bound to a metal ion. We prepared the mono-, bis- and tris-catecholates and -tironates of Fe(3+), V(3+), V(4+)and Mn(3+), the mono- and bis-catecholates and -tironates of Cu(2+), Co(2+), Ni(2+), Zn(2+), Cr(2+) and Mn(2+), and several Ti(4+) and Cr(3+) species. The UV-vis spectra of each complex are described, some of which have not been reported previously. These data can now be applied to characterization of biological metal-catecholate systems.
“…That the Zr−N and N−C bond lengths change by a greater magnitude than the Zr−O and O−C bond lengths upon oxidation suggests that structure A of Scheme is the dominant resonance structure in 2 . This preference for iminephenolate resonance structure A may play a critical role in the stabilization of 2 , by preventing redox disproportionation to release a two-electron-oxidized iminoquinone ligand. , 2 …”
A strategy to enable reactivity analogous to oxidative addition is presented for d 0 transition-metal complexes. The reaction of the redox-active ligand 2,4-di-tert-butyl-6-tert-butylamidophenolate (ap) with ZrCl 4 (THF) 2 affords the new complex Zr IV (ap) 2 (THF) 2 . This compound is formally zirconium(IV) and contains no d electrons; however, exposure of Zr IV (ap) 2 (THF) 2 to chlorine gas results in swift chlorine addition at the zirconium metal center via one-electron oxidation of each ap ligand. The diradical product, Zr IV Cl 2 (isq) 2 (isq ) 2,4-di-tert-butyl-6-tert-butyliminosemiquinone), has been characterized by X-ray crystallography, electron paramagnetic resonance spectroscopy, and SQUID magnetometery.
“…Notably, the corner-shared Mo 2 O 5 core of 2 is a feature of several reported molybdo-organic complexes. − Similar to those metalloorganic complexes, 2 possesses a syn -[Mo 2 O 5 ] 2+ unit with a 2-fold axis of rotation passing through the corner-shared O atom. The bond lengths of the terminal MoO ligands of 2 (1.68–1.69 Å) are in good agreement with MoO bond lengths reported for the related molybdo-organic complexes (1.68–1.71 Å) .…”
Dissolution
of the polyoxometalate (POM) cluster anion H
5
[PV
2
Mo
10
O
40
] (
1
; a
mixture of positional isomers) in 50% aq H
2
SO
4
dramatically enhances its ability to oxidize methylarenes, while
fully retaining the high selectivities typical of this versatile oxidant.
To better understand this impressive reactivity, we now provide new
information regarding the nature of
1
(115 mM) in 50%
(9.4 M) H
2
SO
4
. Data from
51
V NMR
spectroscopy and cyclic voltammetry reveal that as the volume of H
2
SO
4
in water is incrementally increased to 50%,
V(V) ions are stoichiometrically released from
1
, generating
two reactive pervanadyl, VO
2
+
, ions, each with
a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl),
compared to 0.46 V for
1
in 1.0 M aq H
2
SO
4
. Phosphorus-31 NMR spectra obtained in parallel reveal the
presence of PO
4
3–
, which at 50% H
2
SO
4
accounts for all the P(V) initially present
in
1
. Addition of (NH
4
)
2
SO
4
leads to the formation of crystalline [NH
4
]
6
[Mo
2
O
5
(SO
4
)
4
]
(34% yield based on Mo), whose structure (from single-crystal X-ray
diffraction) features a corner-shared, permolybdenyl [Mo
2
O
5
]
2+
core, conceptually derived by acid condensation
of two MoO
3
moieties. While
1
in 50% aq H
2
SO
4
oxidizes
p
-xylene to
p
-methylbenzaldehyde with conversion and selectivity both
greater than 90%, reaction with VO
2
+
alone gives
the same high conversion, but at a significantly lower selectivity.
Importantly, selectivity is fully restored by adding [NH
4
]
6
[Mo
2
O
5
(SO
4
)
4
], suggesting a central role for Mo(VI) in attenuating the (generally)
poor selectivity achievable using VO
2
+
alone.
Finally,
31
P and
51
V NMR spectra show that intact
1
is fully restored upon dilution to 1 M H
2
SO
4
.
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