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Aspects of the organometallic nitrosyl chemistry of Cr, Mo and W Hunter, Allen Dale


The principal organometallic products resulting from the reactions of Na[(ƞ⁵-C₅H₄R)Cr(CO)₃] (R = H or Me) with allyl chlorides in THF are the green, dimeric [(ƞ⁵-C₅H₄R)Cr(CO)₃]₂ complexes (51-67% yields). The red organometallic by-products usually formed during these conversions are novel (ƞ⁶-6-alkenylfulvene)Cr(C0)₃ complexes (5-8% yields) which have been characterized completely by conventional spectroscopic methods. Dark green [W(N0)₂Cl₂]n may be synthesized in high yields by two preparative methods. The first method involves treatment of WCl₆ in CH₂Cl₂ with an excess of NO, and it proceeds via the isolable intermediate complexes, dark violet c̲i̲̲s̲-W(N0)₂Cl₄ and bright green f̲a̲c̲-W(NO)₃Cl₃. The second method involves controlled reaction of W(CO)₆ with two equivalents of ClN0 in CH₂Cl₂. It is initiated by traces of oxidant and probably proceeds via a catalytic, radical-chain mechanism that is described. If either reaction is effected in the presence of two equivalents of CH₃CN, then yellow-green W(N0)Cl₃(CH₃CN)₂ is the only nitrosyl-containing product formed. Polymeric [W(N0)₂Cl₂]n may be cleaved by a variety of Lewis bases, L, and (n̲-Bu)₃Sn(C₅H₅) to form W(N0)₂Cl₂L₂ (L = phosphine, phosphite, CH₃CN, etc.) and CpW(N0)₂Cl (Cp - ƞ⁵-C₅H₅), respectively, in good yields. The synthesis of the electron-rich nitrosyl complexes CpM(NO)L₂ (M = Cr, Mo, or W; L = P(0Me)₃, PMePh₂, P(n̲-Bu)₃, SbPh₃ or 1/2 (dppe)) is described. They are preparable in moderate to high yields by the reduction of the iodo dimers [CpM(NO)In]₂ (M = Cr, n = 1; M = Mo or W, n = 2) with sodium amalgam in THF ln the presence of the appropriate Lewis base, L, and they exhibit metal-dependent trends in vNO (Cr » Mo > W), δ ³¹P (Cr > Mo » W), and ²J₃₁p (Cr < Mo < W). These reduction reactions proceed via a number of transient intermediates, some of which are isolable. A unified mechanism for these reductive syntheses is proposed. The novel complexes, CpMo(NO)(ƞ⁴-trans-diene) (diene = acyclic conjugated diene) and CpMo(NO)(ƞ⁴-c̲i̲s̲-2,3-dlmethyl-butadiene)t are preparable in moderate yields by the reduction of [CpMo(NO)I₂]₂ with sodium amalgam in THF in the presence of the appropriate diene. The reaction between [CpMo(NO)I₂]₂ and C₄H₆•Mg•2(THF) results in the formation of a green, isolable oligomeric complex CpMo(NO)I(ƞ³-C₃H₄R) (where R = CH₂MgI and the nitrosyl oxygen acts as a Lewis base towards Mg) that can be hydrolyzed to CpMo(NO)I(ƞ³-C₄H₇) or converted to CpMo(NO)(ƞ⁴+-t̲r̲a̲n̲s̲-C₄H₆). These diene complexes have been fully characterized by conventional spectroscopic techniques (extensive ¹H and ¹³C NMR spectra being particularly informative) and by single-crystal X-ray structural determinations of CpMo(NO)(ƞ⁴-t̲r̲a̲n̲s̲-2,5-dimethyl-2,4-hexadiene) and CpMo(N0)(ƞ⁴-c̲i̲s̲-2,3-dimethyl-butadlene). A molecular orbital rationale for the structural and spectrocopic properties and relative stabilities of these c̲i̲s̲- and t̲r̲a̲n̲s̲-diene complexes is then presented.

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