Metallacumulenylidene Complexes in the Cycloheptatrienyl Molybdenum Series [Mo{(C)_n═CR₂}L₂(η-C₇H₇)]⁺ (n = 0−2; L₂ = P-Donor Ligand). X-ray Crystal Structures of [Mo{C═C(H)Ph}{P(OMe)₃}₂(η-C₇H₇)][BF₄] and [Mo(C═C═CPh₂)(dppe)(η-C₇H₇)][PF₆] and DFT Investigation of Cumulenylidene Ligand Orientations

Abstract

A series of metallacumulenylidene complexes of the cycloheptatrienyl molybdenum auxiliary [Mo{(C) n CR2}L2(η-C7H7)]+ (n = 0−2; L2 = P-donor ligand) have been synthesized by reaction of [MoXL2(η-C7H7)] (X = Br, L2 = Ph2PCH2CH2PPh2 (dppe); X = I, L2 = 2P(OMe)3) or [Mo(OCMe2)L2(η-C7H7)]+ (L2 = dppe, Ph2PCH2PPh2 (dppm)) with the terminal alkynes HCCR; use of [MoBr(dppe)(η-C7H7)] as a precursor is facilitated by an improved, one-step synthesis from [MoBr(CO)2(η-C7H7)]. Syntheses of the 2-oxacyclocarbene complexes [Mo(CCH2CH2(CH2) x O)L2(η-C7H7)]+ (x = 1, L2 = dppm (1), dppe (2); x = 2, L2 = dppm (3), L2 = dppe (4)), the vinylidenes [Mo{CC(H)(CH2)2CH2OH}(dppe)(η-C7H7)][PF6] (5), [Mo{CC(H)(CH2)xPh}(dppe)(η-C7H7)]+ (x = 2 (6), 1 (7)), and [Mo{CC(H)Ph}{P(OMe)3}2(η-C7H7)]+ (8), and the first examples of monometallic molybdenum allenylidene complexes, [Mo(CCCRPh)(dppe)(η-C7H7)]+ (R = Ph (9), Me (10)) are reported. X-ray structural studies on complexes 8 and 9 have determined Mo−Cα distances as follows: 8, 1.929(3) Å; 9, 1.994(3) Å. In 8, the vinylidene ligand substituents lie in the pseudo mirror plane of the MoL2(η-C7H7) auxiliary (vertical orientation) with the phenyl group located syn to the cycloheptatrienyl ring, whereas the allenylidene ligand substituents of 9 are perpendicular to the pseudo mirror plane (horizontal orientation). This series of atypical orientations of cumulenylidene ligand substituents, imposed by the cycloheptatrienyl molybdenum auxiliary, has been investigated further by variable-temperature NMR spectroscopy and quantum-chemical theoretical calculations. A DFT analysis of the complexes [Mo(CCH2)(dppe)(η-C7H7)]+ (11), [Ru(CCH2)(dppe)Cp]+ (12), and [Mo(CCCMePh)(dppe)(η-C7H7)]+ (10) concurs with experimentally determined cumulenylidene ligand orientations. A separate analysis of the fragments [Mo(dppe)(η-C7H7)]+ and [Ru(dppe)Cp]+ reveals that the HOMO of the [Mo(dppe)(η-C7H7)]+ fragment includes a significant contribution from the metal d z 2 orbital, whereas the HOMO of the [Ru(dppe)Cp]+ fragment is based on a metal d xy orbital, orthogonal to the HOMO of the [Mo(dppe)(η-C7H7)]+ unit. In cumulenylidene complexes of the Mo(dppe)(η-C7H7) auxiliary, interactions between the d z 2 -based HOMO of the metal fragment and the vacant LUMO of the cumulenylidene ligand dominate the control of ligand orientation and thus account for the observed structures.

Synthesis, structure and redox chemistry of the aminoallenylidene complex [Mo{C C C(Me)NEt2}(dppe)(η-C7H7)][BPh4]

“…Consistent with the enhanced alkynyl character of the aminoallenylidene ligand of [1] + , the position of the ν(CCC) band is to high wavenumber of the cumulenic diphenylallenylidene analogue [2] + , although it is still much lower in wavenumber than the ν(C≡C) stretch of a typical alkynyl complex [Mo(C≡CR)(dppe)(η-C 7 H 7 )] (ν(C≡C) in the range 2040-2060 cm -1 ). [16b] The 31 P{ 1 H} NMR chemical shift of the dppe ligand phosphorus atoms also appears to be sensitive to the alkynyl character of [1] + with the chemical shift of 60.3 ppm, quite close to typical values determined for alkynyl complexes (generally in the range 64-66 ppm) [16b], and distinct from cumulenylidene complexes such as[2] + and the vinylidenes [Mo{C=C(H)R}(dppe)(C 7 H 7 )] + which exhibit 31 P{ 1 H} NMR shifts in the range 51-54 ppm [17]. The ν(C γ =N) stretching frequency at 1542 cm -1 is in the correct region for the C=N multiple bond of aminoallenylidenes and, in common with related complexes based on the {RuCl(dppm) 2 } moiety [9c], restricted rotation about the aminoallenylidene C=N bond results in inequivalence of the amino ethyl substituents in both 1 H and 13 C{ 1 H} NMR spectra.…”

“…A few examples of stable 17-electron dications of the type [MoX(dppe)(η-C 7 H 7 )] 2+ are known [16a], including the heteroatom substituted cyclic oxacarbene [Mo{C(CH 2 ) 3 O}(dppe)(η-C 7 H 7 )] 2+ and this suggested that a related heteroatom substituted allenylidene could also exhibit the requisite stability. The diphenylallenylidene complex [Mo(C=C=CPh 2 )(dppe)(η-C 7 H 7 )][PF 6 ] has been prepared previously [17] via the classical method of Selegue but in the current work, the focus was upon the development of the synthesis of a heteroatom substituted derivative [Mo{C=C=C(R)NR´2}(dppe)(η-C 7 H 7 )] + for which the redox potential for one-electron oxidation may be expected to be significantly more thermodynamically favourable due to the enhanced contribution of alkynyl resonance forms (III) and (IV) to the structure.…”

Ruthenium Acetate Complexes as Versatile Probes of Metal–Ligand Interactions: Insight into the Ligand Effects of Vinylidene, Carbene, Carbonyl, Nitrosyl and Isocyanide

“…For example, the 13 C{ 1 H} NMR spectra of the complexes exhibited resonances at δ = 304.7 (t, 2 J PC = 11.8 Hz), δ = 306.8 (t, 2 J PC = 11.7 Hz) and δ = 311.0 ppm (t, 2 J PC = 11.8 Hz) for 11a , 11b and 11c , respectively. An analysis of the chemical shift of a number of oxacyclocarbene ligands (Table 1) demonstrates that this trend of increasing chemical shift with increasing ring size appears to be present in a number of species of this type 20…”

Electronic structure and spectroscopy of the cycloheptatrienyl molybdenum halide complexes [MoBrL2(η-C7H7)]+ (L2= 2CO, n= 0; L2= 2,2′-bipyridyl, n= 0 or 1)

“…The importance of metal-ring δ-interactions in the electronic structure of cycloheptatrienyl metal-sandwich complexes is well established [2-4, 8, 15] but very little attention has been given to the extension of this principle to half-sandwich systems [16] such as [ML 3 (η-C 7 H 7 )] n+ where potentially the impact on the tripodal L 3 ligand set could be even more pronounced. In this context, a recent series of investigations on half-sandwich vinylidene [Mo(C=CHR)(dppe)(η-C 7 H 7 )] + [17] (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) and carbon chain complexes [Mo{(C≡C) x -C≡CR)L 2 (η-C 7 H 7 )] n+ (n = 0 or 1; x = 0 or 1, L 2 = dppe; x = 1, L 2 = bpy) [18,19], has demonstrated a re-ordering of the d-orbital manifold arising from metal-ring δ-bonding, leading to a metal-centred HOMO with significant d z 2…”

“…A further distinction between C 7 H 7 and cyclopentadienyl ligands is a difference in formal charge of the co-ordinated ligand. Although the assignment of a formal charge to the C 7 H 7 ligand, ranging from +1 to −3, remains an issue of debate [3b, 7a, 9, 10, 22], the positive charge formalism, C 7 H 7 + , may have some validity in the half-sandwich series [ML 3 (η-C 7 H 7 )] n+ (M = Cr, Mo or W) for which selected examples exhibit properties typical of d 6 metal centres [17]. On this basis, it has been suggested that the cycloheptatrienyl ligand can act as a good acceptor group in MLCT and ILCT /LLCT processes [23,24].…”