The dihalide capped trinuclear nickel clusters
Ni3(μ3-X)2(μ2-dppm)3
(X = I (1a), Br (1b); dppm =
Ph2PCH2PPh2) were synthesized and converted to
their respective monocations
[Ni3(μ3-X)2(μ2-dppm)3]+
(X = I (1a
+
),
Br (1b
+
)) via a single
electron oxidation. Clusters 1a and
1a
+
were characterized by X-ray
crystallography.
Displacement of a triply-bridging iodide ligand in
Ni3(μ3-I)2(μ2-dppm)3
(1a) by π-acceptor ligands produces a class
of 48-electron trinuclear nickel clusters of the general formula
[Ni3(μ3-L)(μ3-I)(μ2-dppm)3]+
(L = CO (2a); CNR, R
= CH3 (3a),
2,6-(CH3)2C6H3
(4a), i-C3H7
(5), C6H11 (6),
t-C4H9 (7),
CH2C6H5 (8),
C6H5 (9),
p-C6H4I (10),
p-C6H4Br (11), p-C6H4Cl
(12), p-C6H4F
(13),
p-C6H4CH3
(14),
p-C6H4CF3
(15),
p-C6H4OCH3
(16), and p-C6H4CN
(17)).
A similar but less extensive study was conducted with the
dibromide capped cluster 1b. The X-ray crystal
structure
of cluster 2a, as the PF6
- salt,
was also obtained. Clusters 2−17 possess
strikingly similar spectroscopic and
electrochemical properties. This is ascribed to the lack of
interaction between the a
2
LUMO of
clusters 2−17 and
the molecular orbitals of the capping π-acceptor ligands. The
unexpected appearance of two ν(C⋮N) bands in
the
FT-IR spectra of clusters 3−17 was demonstrated
to be the result of a Fermi resonance involving the
ν(C⋮N)
fundamental and the first overtone of the ν(N−C(alkyl))
fundamental of the capping isocyanide. In addition,
molecular
orbital calculations on 1−17 provide insights
into the differences in the physical properties and reactivities of
clusters
of this class capped by π-donor (1) or π-acceptor
ligands (2−17).
Described are studies directed toward the chemical research and development of an alternative synthesis to 9, the penultimate intermediate of clofencet (1), a novel wheat-hybridizing agent. Retrosynthetic analyses as well as the results obtained from feasibility studies are detailed, leading to the successful development of an alternative process. The key features of the novel route are a method for preparing on-scale ethyl diazoacetate (28) in a safe and effective manner, and the Lewis acidcatalyzed reaction of 28 with hydrazonoacetaldehyde 29, affording β-ketoester 30. The synthesis is completed via propionylation of 30, acid-catalyzed cyclization of 31 to pyridazinecarboxylic acid ester 32, followed by saponification and isolation of carboxylic acid 9. The results and challenges of eight pilot-plant runs are reported. The baseline process developed produced over 45 kg of 9 in 43-45% yield.
Variable-temperature two-dimensional phase-sensitive 31P{1H} EXSY NMR spectroscopy
was used to determine the kinetics and thermodynamics of terminal ligand redistribution
in the nickel A-frames Ni2(dppm)2(CCH2)X2 (X = SCN (1), Cl (3); dppm = bis(diphenylphosphino)methane). Solutions containing 1:1 mixtures of 1 and 3 undergo terminal ligand
exchange via a two-step process involving a mixed-ligand intermediate, Ni2(dppm)2(CCH2)(Cl)(SCN) (2). This type of terminal ligand exchange is characteristic of this class of
compounds. One-dimensional 31P{1H} NMR spectra for the mixed-halogen or halogen/pseudo-halogen species show a strong solvent dependence. The observed NMR spectra for 2 range
from a broad singlet (DMSO) to a well-resolved AA‘BB‘ multiplet (benzene) and are dependent
on the rate of ligand substitution. Rate constants for the terminal ligand exchange processes
in DMSO-d
6 were calculated by three methods from the 31P{1H} EXSY NMR data. From
these measurements, a complete thermodynamic characterization of this exchange process
was achieved. Entropies of activation calculated from Eyring plots range from −11.4 to
−19.0 cal/(K mol). This implies ordering of the system during the transition state and is
consistent with an associative interchange (Ia) mechanistic model. Free energy calculations
show an intrinsic thermodynamic stability associated with the mixed-ligand species 2.
The reaction of bis(cyclooctadiene)nickel(0) with bis(dimethylphosphino)methane (dmpm) and 1,1-dichlorovinylidene in THF yields the vinylidene-bridged binuclear nickel A-frame complex [Ni 2 (µ-CdCH 2 )(dmpm) 2 Cl 2 ] (1a). The structure of 1a was determined by X-ray crystallography. The Ni-Ni separation is 2.898(2) Å, and the vinylidene carbon-carbon bond distance is 1.35(2) Å. The structure of the bis(diphenylphosphino)methane (dppm) bridged µ-vinylidene complex [Ni 2 (µ-CdCH 2 )(dppm) 2 Br 2 ] (2b) was also determined by X-ray crystallography. The Ni-Ni separation of 2.874(2) Å for 2b is comparable to that of 1a. The series of µ-vinylidene nickel A-frames with substituted terminal ligands [Ni 2 (µ-CdCH 2 )(PR 2 CH 2 PR 2 ) 2 X 2 ] (X ) Cl, Br, I, R ) Me; X ) Cl, Br, I, NCS, OCN; R ) Ph) were prepared and characterized. The nickel A-frames with different bridging ligands [Ni 2 (µ-CdE)(dppm) 2 Cl 2 ] (E ) O, NPh) were also examined. Extended Hu ¨ckel molecular orbital (EHMO) calculations indicate that the HOMO in these nickel A-frame complexes is primarily metal based and that the LUMO is derived primarily from the π* system of the µ-CdCH 2 ligand. The σ-donating ability of the diphosphine ligands affects the electronic structure of these complexes primarily by stabilizing interactions between the filled d-orbital manifold and the b 2 orbital of the vinylidene fragment.
Low-temperature FTIR studies were conducted to probe the mechanism of the Fe(CO) 5catalyzed [4 + 1] cyclization of allenyl ketones and CO to form R-alkylidenebutenolides. Photolysis of Fe(CO) 5 in 2-methyltetrahydrofuran (2MeTHF) at 190-230 K produced an iron carbonyl solvento species, Fe(CO) 4 (2MeTHF). When the matrix was warmed, Fe(CO) 5 was regenerated quantitatively. Photolysis of Fe(CO) 5 (in 2MeTHF) at 230 K in the presence of 5-methyl-3,4-hexadien-2-one under similar conditions gave Fe(CO) 4 (2MeTHF) initially. Warming the samples regenerates a portion of the Fe(CO) 5 while a new band appears at 1767 cm -1 corresponding to the R-alkylidenebutenolide 11. Kinetic experiments at 243 K show no dependence upon CO or allenyl ketone. Reactions in pyridine gave significantly slower rates than THF. This is consistent with rate-determining dissociation of the coordinated solvent to form an unsaturated iron center.
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