Herein we report the synthesis and characterization of anionic boron-and carbon-based Kekulédiradicaloids spanned by a p-phenylene bridge. In contrast to Thiele's hydrocarbon, a closed-shell singlet system, they show an appreciable population of the triplet state at room temperature, as evidenced by both NMR and EPR spectroscopy. Moreover, en route to these anionic boronand carbon-based hetero-diradicaloids, the formation of an isolable diamino(4-diarylboryl-phenyl)methyl radical was observed.
Ligand design represents a central
tenet of synthetic chemistry,
wherein simple modification can lead to major differences in reactivity.
Herein, we describe the preparation of two bis(diphosphino)nickel(II)
hydride complexes that contain eight pendant boranes in their secondary
coordination sphere, [Ni(H)(P2BR
4)2]+ (R = Cy or Mes; Mes = 2,4,6-trimethylphenyl).
Divergent reactivity of the cyclohexyl analogue toward the [NAD]+ model, 3-acetyl-N-benzylpyridinium bromide
([BNAcP]Br), is underscored. While [Ni(H)(P2BCy
4)2]+ undergoes rapid hydride transfer,
the related species [Ni(H)(dnppe)2]+ [dnppe
= 1,2-bis(di-n-propylphosphino)ethane] and adduct
[Ni(H)(P2BCy
4)2(DMAP)8]+ (DMAP = 4-N,N-dimethylaminopyridine) exhibit no such reactivity. This borane-appended
nickel(II) hydride distinguishes itself from its “all-alkyl”
cousins and provides future opportunities for the design of [Ni(H)(diphosphine)2]+ reagents for hydride transfer.
Trinuclear complexes of group 6, 8, and 9 transition metals with a (μ3 -BH) ligand [(μ3 -BH)(Cp*Rh)2 (μ-CO)M'(CO)5 ], 3 and 4 (3: M'=Mo; 4: M'=W) and 5-8, [(Cp*Ru)3 (μ3 -CO)2 (μ3 -BH)(μ3 -E)(μ-H){M'(CO)3 }] (5: M'=Cr, E=CO; 6: M'=Mo, E=CO; 7: M'=Mo, E=BH; 8: M'=W, E=CO), have been synthesized from the reaction between nido-[(Cp*M)2 B3 H7 ] (nido-1: M=Rh; nido-2: M=RuH, Cp*=η(5) -C5 Me5 ) and [M'(CO)5 ⋅thf] (M'=Mo and W). Compounds 3 and 4 are isoelectronic and isostructural with [(μ3 -BH)(Cp*Co)2 (μ-CO)M'(CO)5 ], (M'=Cr, Mo and W) and [(μ3 -BH)(Cp*Co)2 (μ-CO)(μ-H)2 M''H(CO)3 ], (M''=Mn and Re). All compounds are composed of a bridging borylene ligand (B-H) that is effectively stabilized by a trinuclear framework. In contrast, the reaction of nido-1 with [Cr(CO)5 ⋅thf] gave [(Cp*Rh)2 Cr(CO)3 (μ-CO)(μ3 -BH)(B2 H4 )] (9). The geometry of 9 can be viewed as a condensed polyhedron composed of [Rh2 Cr(μ3 -BH)] and [Rh2 CrB2 ], a tetrahedral and a square pyramidal geometry, respectively. The bonding of 9 can be considered by using the polyhedral fusion formalism of Mingos. All compounds have been characterized by using different spectroscopic studies and the molecular structures were determined by using single-crystal X-ray diffraction analysis.
The syntheses and structural characterization of hypo-electronic di-molybdenum triple-decker sandwich clusters are reported. Thermolysis of [Ru3(CO)12] with an in situ generated intermediate obtained from the reaction of [Cp*MoCl4] with [LiBH4·THF] yielded an electron deficient triple-decker sandwich complex, [(Cp*Mo)2{μ-η(6):η(6)-B4H4Ru2(CO)6}], . In an effort to generate analogous triple-deckers containing group-16 elements, we isolated [(Cp*Mo)2{μ-η(6):η(6)-B4H4ERu(CO)3}] (: E = Te; : E = S; : E = Se). These clusters show a high metal coordination number and cross cluster Mo-Mo bond. The formal cluster electron count of these compounds is four or three skeletal electron pairs less than required for a canonical closo-structure of the same nuclearity. Therefore, these compounds represent a novel class of triple-decker sandwich complex with 22 or 24 valence-electrons (VE), wherein the "chair" like hexagonal middle ring is composed of B, Ru and chalcogen. One of the key differences among the synthesized triple-decker molecules is the puckering nature of the middle ring [B4RuE], which increases in the order S < Se < Ru(CO)3 < Te. In addition, Fenske-Hall and quantum-chemical calculations with DFT methods at the BP86 level of theory have been used to analyze the bonding of these novel complexes. The studies not only explain the electron unsaturation of the molecules, but also reveal the reason for the significant puckering of the middle deck. All the compounds have been characterized by IR, (1)H, (11)B, and (13)C NMR spectroscopy in solution and the solid state structures were established by crystallographic analysis.
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