Yu.V. Skripkin

Synthesis and molecular structure of niobocene(tricarbonyl)(π-cyclopentadienyl)molybdenum with metalmetal bond and unusual coordination of carbonyl groups

Abstract (C5H5)2NbBH4 reacts with C5H5M(CO)3Me in toluene solution in the presence of Et3N to give binuclear complexes (C5H5)2NbM(CO)3C5H5 where M is Mo or W (IV and V, respectively). The structure of IV has been studied by X-ray diffraction (the crystals are orthorhombic, a 12.748(5), b 16.745(6), c 14.314 A/ac> ;; Z = 8, space group of Pbca, automatic difractometer Syntex P2I, λ(Mo-Kα, 1382 reflections, R = 0.056, Rw = 0.058). Molecule IV contains a wedge-like sandwich (π-C5H5)2Nb (NbC 2.37–2.48, CC (av) 1.42 A/ac> ;, angle between ring planes 49°) linked with the (π-C5H5)Mo(CO) fragment by a direct NbMo bond (3.073 A/ac> ;) and two bridging CO groups, one nonsymmetrically bonded through the carbon atom only (CO 1.17, NbC 2.53, MoC 2.02 A/ac> ;) and the other σ-bonded to Mo (MoC 1.944 A/ac> ;) and π-bonded to Nb (CO 1.22, NbC 2.22, NbO 2.26 A/ac> ;). Three types of carbonyl groups present in IV give rise to strong IR bands at 1870, 1700 and 1560 cm−1 assigned to the terminal, μ-bridging and σ, π-bridging CO groups respectively. Complex IV has a similar structure. The electronic structure of IV and its dissociation across the NbMo bond are discussed.

The structure and formation of niobocene carbonyl heteronuclear derivatives. The molecular structures of Cp2Nb(CO)(μ-CO)Mn(CO)4, Cp2Nb(CO)(μ-H)Ni(CO)3 and [Cp2Nb(CO)(μ-H)]2Mo(CO)4

Abstract The heteronuclear Cp 2 Nb(CO)(μ-CO)Mn(CO) 4 (I), Cp 2 Nb(CO)(μ-H)Ni(CO) 3 (II) and [Cp 2 Nb(CO)(μ-H)] 2 M(CO) 4 (III, M = Mo;IV, M = W) complexes were prepared by reaction of Cp 2 NbBH 4 /Et 3 N with Mn 2 (CO) 10 in refluxing toluene, direct reaction of Cp 2 NbBH 4 with Ni(CO) 4 in ether, and reaction of Cp 2 NbBH 4 /Et 3 N with M(CO) 5 . THF complexes (M = Mo or W) in THF/benzene mixture. An X-ray investigation of compounds I–III was performed. It is established that in I the bonding between Mn(CO) 5 and Cp 2 Nb(CO) (with the angle (α) between the ring planes being 44.2(5)°) fragments takes place via a direct NbMn bond (3.176(1) A) and a highly asymmetric carbonyl bridge (MnC co 1.837(5) A, NbC co 2.781(5) A). On the other hand, in complex II the sandwich Cp 2 Nb(CO)H molecule (angle α = 37.8°) is combined with the Ni(CO) 3 group generally via a hydride bridge (NbH 1.83 A, NiH 1.68 A, NbHNi angle 132.7°) whereas the large Nb⋯Ni distance, 3.218(1) A, shows the weakening or even absence of the direct NbNi bond. Similarly, in complex III two Cp 2 Nb(CO)H molecules (with α angles equal to 41.4 and 43.0°, respectively) are joined to the Mo(CO) 4 group via the hydride bridges (NbH 1.83 and 1.75 A and MoH 2.04 and 2.06 A) producing a cis -form. The direct NbMo bonds are probably absent, since the Nb⋯Mo distances are rather long (3.579 and 3.565 A). The effect of electronic and steric factors on the structure of heteronuclear niobocene carbonyl derivatives is discussed.

Synthesis and molecular structure of the niobocene sulphide dimer

Abstract The reaction of Cp 2 NbBH 4 in benzene in the presence of Et 3 N and PhSH results in a green, crystalline, diamagnetic complex, [Cp 2 NbSPh] n (I), which probably has a polymer structure. The reaction of Cp 2 NbBH 4 under the same conditions with t-BuSH or (CpCrSCMe 3 ) 2 S yields a red, crystalline, diamagnetic complex, [Cp 2 NbS] 2 (II), whose structure was established by X-ray analysis. In II, the Nb atom belongs to the niobocene fragment, whose inclined rings (dihedral angle 47.25°) are in a staggered conformation (NbC Cp(av.) 2.452(7), CC Cp(av.) 1.40(1) A). Moreover, each Nb atom forms two equivalent short bonds, NbS (2.429(2) and 2.427(2) A), and the angles NbSNb (83.51(5)°) are coequal, as are the SNbS angles (96.49(6)°). The NbNb distance (3.2343(8) A) is evidently bond-forming. The molecule on the whole resembles C 2ν in symmetry.

Synthesis and molecular structure of niobocene trimethylacetate and its π-complex with diphenylacetylene

Abstract Niobocene trimethylacetate Cp 2 Nb(OOCCMe 3 ) (I) does not react with usual n -donors (pyridine and triphenylphosphine), but readily adds a π-acceptor molecule of diphenylacetylene (tolane) in benzene to form Cp 2 Nb(OOCCMe 3 )(π-Ph 2 C 2 ) · 0.5 C 6 H 6 (II). The structures of the diamagnetic complexes I and II have been determined by an X-ray diffraction study. These molecules represent wedge-like sandwiches wit dihedral angles between cyclopentadienyl ligands equal to 44.4 and 50.7°, and average NbC distances of 2.39 and 2.44 A, respectively. The bisector plane of I contains the chelate trimethylacetate group (NbO bond lenghts 2.23 and 2.24 A) and that of II contains the coordinated tolane molecule and the oxygen atom of the terminal trimethylacetate ligand (NbO 2.16, NbC 2.18 and 2.19, CC 1.29 A, PhCC angles 141 and 146°). An unusually large splitting of OCO stretching frequencies is observed in the IR spectrum of I (1652−1305 = 347 cm −1 ). Structural characteristics of the coordinated CC triple bond in II are similar to those found in Cp(π-Ph 4 C 4 )Nb(CO)(π-Ph 2 C 2 ) studied earlier. The role played by the Nb III lone pair in I and II is discussed.

The importance of steric factors in the formation and structures of heterobinuclear wedge-like sandwich derivatives. The crystal and molecular structure of niobocenehydrido-(σ,π-cyclopentadienylidene)iron(dicarbonyl)(Nb—fe)

Abstract CpFe(CO) 2 CH 3 reacts with Cp 2 NbH made from Cp 2 NbBH 4 and Et 3 N to give Cp 2 NbH(μ-C 5 H 4 )F e (III). As shown by X-ray diffraction, III contains the Cp 2 NbH sandwich fragment with a 46.8° angle between the rings linked with the dicarbonyliron moiety by the NbFe bond (2.968 A), observed for the first time, and a cyclopentadienyl bridge C 5 H 4 , involving the NbC. σ-bond (2.189 A) and C 5 H 4 Fe π-bond (2.085 A). A probable reaction scheme leading to III and general patterns of formation of other heterobinuclear derivatives of sandwich complexes Cp 2 MLM′(L′) n are discussed. The importance of steric effects due to nonbonded interligand interactions between the M′(L′) n fragment and the sandwich system is emphasized. Increase of steric strain in the binuclear system facilitates its unusual transformations.

Synthesis and properties of cyclopentadienylniobium trimethylacetates

Abstract Reaction of Cp2NbBH4 with Me3CCOOH gives the diamagnetic monomerical Cp2NbOOCCMe3, which contains a terminal carboxyl and reacts readily with an equimolar amount of CMe3COOH to give the paramagnetic monomerical Cp2Nb(OOCCMe3)2 containing terminal carboxyls and, probably, non-equivalent cyclopentadienyl ligands. The latter compound easily loses two C5H5 ligands under the action of two moles of Me3CCOOH, giving the paramagnetic monomerical Nb(OOCCMe3)4 in which the carboxylate groups are chelated and the niobium coordination number is eight. Refluxing a 1 : 5 Cp2NbBH4/Me3CCOOH mixture in xylene leads to the diamagnetic pentacarboxylate (Nb(OOCCMe3)5)n (probably a dimer).

Comparison of the molecular structures and infrared spectra of titanocene and niobocene chelate trimethylacetates

An X-ray structural study (λ(Mo-K α ), θ2θ scanning, 2θ ⩽ 46°, T = −120°C) of Cp 2 TiOOCCMe 3 (I) has been performed. The cyclopentadienyl ligands form π-bonds with the Ti atom (mean TiC mean 2.40(1) A, mean CC 1.48(2) A, the dihedral angle between the rings being 45(1)°. The chelate OOCCMe 3 ligands is located in a bisecting plane with equivalent TiO bonds equal to 2.13(1) A (Ti⋯C OCO 2.51(1) A, OCO angle 116(1)°). The geometry of I has been compared to the previously investigated structure of Cp 2 NbOOCCMe 3 (II) which has the carboxylate group farther from the metal atom (NbO 2.23(1) A, Nb⋯C OCO 2.61(1) A, OCO angle 118(1)°). The effect of d 1 and d 2 metal electronic configurations on the geometry and IR spectra of I and II is discussed in terms of the nature of the MOCO bonding.

Synthesis and molecular structure of a pentanuclear complex with niobium-indium bonds observed for the first time

The reaction of InCl~ with (MeC~H~)=NbBH~, which is activated by treatment with BuLi in the presence of Et~N, has been studied by us. The reaction occurs in tetrahydrofuran (THF) with the formation of the complex [(MeC~H~)NbH]~In=CI= (I) which was isolated as orange-yellow crystals by recrystallization from toluene and THF (48% yield). Complex (I) was diamagnetic. Found: C 39.70; H 4.76; C1 6.03%. C~H~Nb~In=CI~. Calculated: C 40.87; H 4.25; C1 6.71%. The structure of (I) was established by x-ray structural analysis of crystals of (I). THF solvate was obtained by the slow crystallization of (I) from THF (see Fig~ I)~