Carbene complexes. Part 9. Electron-rich olefin-derived carbene–molybdenum(0) and amidinium molybdate(0) complexes, and the crystal and molecular structure of cis-tetracarbonylbis(1,3-dimethylimidazolidin-2-ylidene)molybdenum(0), cis-[Mo(CO)4{CN(Me)CH2CH2NMe}2]

Abstract

Carbene–Mo^o complexes are obtained by the thermal reaction of an electron-rich olefin [graphic omitted]R]₂, L^R₂, with (a)[Mo(CO)₆]{yielding [Mo(CO)₅L^R] or cis-[Mo(CO)₄(L^R)₂](R = Me, Et, or PhCH₂)(the tricarbene complex is unstable)}, and (b)[Mo(CO)₂(C₅H₅-η)NO]{yielding [Mo(CO)(C₅H₅-η(L^R)NO](R = Me or p-tol)}. By contrast, with [Mo(CO)₃(C₅H₅-η)H] or [{Mo(CO)₃(C₅H₅-η)}₂], the olefin reacts as a reducing agent, yielding the amidinium molybdate(0) complexes [HL^R]⁺[Mo(CO)₃(C₅H₅-η)]^– or [L^R₂]²⁺[Mo(CO)₃(C₅H₅-η)]^–₂(R = Me). The six-membered chelate olefin [C[graphic omitted]Me]₂, L′^Me₂, behaves qualitatively in a similar fashion, but is less reactive, dicarbene–Mo^o complexes not being accessible. The cis-dicarbene–Mo^o complexes are isomerised photochemically to the trans-complexes, but the former are thermodynamically the more stable. Reaction of [Mo(CO)₅L^Et] with L^Me₂ yields cis-[Mo(CO)₄(L^Et)(L^Me)], whereas [Mo(CO)₅L^Me)] with L^Et₂ affords also cis-[Mo(CO)₄(L^Me)₂] and cis-[Mo(CO)₄(L^Et)₂]. The redistribution reaction cis-[Mo(CO)₄(L^R)₂]+[Mo(CO)₆]⇌ 2[Mo(CO)₅L^R](R = Me or Et) is reversible. Other reactions of monocarbene complexes give cis-[Mo(CO)₄(L^R)Q][Q = C₅H₅N, PPh₃, or P(C₆H₁₁)₃, fac-[Mo(CO)₃(L^R)Q′₂][Q′₂={P(OMe)₃}₂ or diphos], or cis-[Mo(CO)₄{C(OMe)Me}L^R]. ν(CN₂) is at 1 510—1 480 cm^–1 for the carbene–Mo^o complexes, but at 1 700–1 640 cm^–1 for the amidinium cations, and ν(CO) values are very low; ¹H n.m.r. spectra provide information on conformational aspects; ΔG^‡ for Mo–C_carb rotation in cis-[Mo(CO)₄(L^R)₂] is ca. 10 kcal mol^–1. ¹³C Chemical shifts for C_carb. and CO are comparable (but are distinguished by ¹H decoupling and ¹H off-resonance studies). Both are in the range 230–210 p.p.m. upfield from SiMe₄, whereas for [HL^Me]⁺ or [L^Me₂]²⁺ there is a further 50–60 p.p.m. upfield shift for δ[C_{carbonium ion}]. A single-crystal X-ray analysis of (4)cis-[Mo(CO)₄(L^Me)₂] shows octahedral Mo, with MO–CO mutually-trans bond lengths longer by 0.048(2)Å than MO–CO trans to carbene [2.024(3) and 2.032(3)Å]; Mo–C_carb[2.293(3)Å] is appropriate for single bonds. The dihedral angles between the carbene ligand planes and the co-ordination planes in which they are involved are ca. 45°, which is sterically the preferred conformation. It is concluded that the carbene ligand is a strong σ-donor but a poor π-acceptor.