•The aminoallenylidene [Mo{CCC(Me)NEt2}(dppe)(η-C7H7)][BPh4] has been synthesised.•Cumulenic and Iminium Alkynyl resonance forms contribute to the overall structure.•Reversible one-electron oxidation to the radical dication is observed.•The allenylidene ν(CCC) stretch shifts to higher wavenumber following oxidation.The reaction of [MoBr(dppe)(η-C7H7)] (dppe = Ph2PCH2CH2PPh2) with HCCCCSiMe3 and Na[BPh4] in 1:1 NHEt2/THF as solvent yields the aminoallenylidene complex [Mo{CCC(Me)NEt2}(dppe)(η-C7H7)][BPh4], [1][BPh4]. The reaction likely proceeds via nucleophilic addition of NHEt2 at Cγ of a butatrienylidene intermediate. Structural and spectroscopic characterisation of [1][BPh4] indicate a significant contribution of an iminium alkynyl resonance form to the overall structure of the heteroatom stabilised allenylidene ligand. The X-ray structural study of [1][BPh4] determines a MoCα bond length of 2.077(3) Å, intermediate between that of the cumulenic diphenylallenylidene analogue [Mo(CCCPh2)(dppe)(η-C7H7)][PF6] (1.994(3) Å) and the alkynyl compound [Mo(CCPh)(dppe)(η-C7H7)] (2.138(5) Å). Complex [1][BPh4] undergoes a reversible one-electron oxidation with E½ = −0.19 V with respect to the FeCp2/FeCp2+ couple and the stable 17-electron radical dication [1]2+ is readily observed by spectroelectrochemical methods. IR spectroelectrochemistry in CH2Cl2 demonstrates that the ν(CCC) stretch, characteristic of the allenylidene ligand, shifts to higher wavenumber (from 1959 to 2032 cm−1) as a result of oxidation of [1]+ to [1]2+, consistent with a strongly metal-centred redox process and an enhancement in the alkynyl character of the allenylidene ligand following one-electron oxidation.Spectroelectrochemical IR investigations on the aminoallenylidene complex [Mo{CCC(Me)NEt2}(dppe)(η-C7H7)][BPh4], [1]+, indicate that a reversible one-electron oxidation to the 17-electron radical dication [1]2+ results in an enhancement in the iminium alkynyl character of the ligand.

•Conversion of a vinylidene to an alkoxycarbene at a Fe(dppe)Cp centre is reported.•The methoxycarbene [Fe{C(OMe)Me}(dppe)Cp]+ has been structurally characterised.•Fe-Cα distances for a range of heteroatom stabilised carbene ligands are compared.The reaction of [FeI(dppe)Cp], 1 (dppePh2PCH2CH2PPh2) with HC≡CSiMe3 in refluxing methanol results in formation of a mixture of the vinylidene complex [Fe(CCH2)(dppe)Cp]+, 2 and the heteroatom stabilised alkoxycarbene [Fe{C(OMe)Me}(dppe)Cp]+, 3. The carbene 3 is formed by addition of the alcohol solvent to the reaction intermediate vinylidene 2, and prolonged reflux in methanol (4 days) resulted in complete conversion of 2 to 3. The methoxycarbene, 3 was isolated as a stable solid and fully characterised by spectroscopic and structural methods. Vinylidene 2 also reacts with ethanol to give the ethoxycarbene [Fe{C(OEt)Me}(dppe)Cp]+, 4 but no reaction was observed in refluxing isopropanol, thus providing a specific synthesis of vinylidene 2 from 1 and HC≡CSiMe3. An example of an alcohol based, intramolecular nucleophilic addition to a mono-substituted vinylidene [Fe(CCHR)(dppe)Cp]+ was uncovered in the reaction of 1 with 3-butyn-1-ol which leads to the synthesis of the cyclic oxacarbene complex [Fe{C(CH2)3O}(dppe)Cp][PF6], 5. X-ray crystallographic investigations on 3[I] and 5[PF6] determine Fe-Cα bonds lengths of 1.851(5) and 1.834(2) Å respectively, intermediate between previously reported Fe-Cα distances for the amino carbene [Fe{C(NH2)Me}(dppe)Cp][PF6] and the alkylidene complex [Fe{C(H)Me}(dppe)Cp*][PF6].The vinylidene complex [Fe(CCH2)(dppe)Cp]+ reacts with methanol under prolonged reflux to give the heteroatom stabilised, methoxycarbene complex [Fe{C(OMe)CH3}(dppe)Cp]+ which has been structurally characterised.

DFT calculations at the B3LYP/Def2-SVP level have been conducted on the half-sandwich cycloheptatrienyl molybdenum complexes [Mo(CO)3(η-C7H7)]+, [1]+ and [MoBrL2(η-C7H7)]n+ (L2 = 2 CO, n = 0, 2; L2 = bpy, n = 0, 3; L2 = bpy, n = 1, [3]+; bpy = 2,2′-bipyridyl). In all cases, strong δ-bonding interactions operate between the e2 level of the C7H7 ring and metal dxy   and dx2-y2dx2-y2 orbitals resulting in a metal-centred HOMO with substantial dz2dz2 character in the 18-electron, closed shell systems. The experimental electronic UV–Vis spectra of [1]+, 2 and 3 are accurately reproduced by TD-DFT methods. For complexes 2 and 3, assignments made with the assistance of calculated spectra indicate that absorptions in the region 390–770 nm originate from a series of MLCT (metal–ligand charge transfer) or ILCT (inter-ligand charge transfer) transitions in which carbonyl, C7H7 and 2,2′-bipyridyl ligands act as acceptor systems from the metal or mixed metal and bromide donor groups. The metal-centred, one-electron oxidation of 3 to 3[PF6] results in almost complete quenching of the visible region MLCT/ILCT absorptions of 3 and replacement with weak transitions probably arising from bromide to metal LMCT (ligand to metal charge transfer) processes.Electronic structure calculations (DFT, B3LYP/Def2-SVP) on the half-sandwich cycloheptatrienyl molybdenum complexes [MoBrL2(η-C7H7)] (L2 = 2 CO or 2,2′-bipyridyl) reveal that in each case the HOMO features significant metal dz2dz2 character but the composition of the LUMO is L2 dependent. TD-DFT methods have been employed to analyse the experimental UV–Vis electronic absorption spectra of these complexes.

•A large scale synthesis of [FeI(dppe)Cp] requiring only thermal initiation is reported.•A large scale synthesis of [MoBr(dppe)(η7-C7H7)] has been developed.•31P{1H} NMR demonstrates Br/Cl halide exchange of [MoBr(dppe)(η7-C7H7)] in CH2Cl2.Improved synthetic routes to [FeI(dppe)Cp], and [MoX(dppe)(η7-C7H7)] (X = Br, Cl) are described. [FeI(CO)2Cp] reacts with dppe in refluxing toluene to give multi-gram quantities of [FeI(dppe)Cp], 3 without the need for photochemical activation. The direct thermal reaction of [MoX(CO)2(η7-C7H7)], 4 with dppe is complicated by competition between associative and dissociative substitution processes. Where X = Br, the associative pathway, which features [MoBr(CO)2(dppe)(η3-C7H7)], 5 as an intermediate, yields cationic [Mo(CO)(dppe)(η7-C7H7)]Br, 7 as the major product. However conditions which promote a dissociative mechanism give good yields of [MoX(dppe)(η7-C7H7)], 6 albeit contaminated with small quantities of [Mo(CO)4(dppe)], 8. The separation of 8 from 6 can be effected by chromatography on alumina but this procedure activates 6 to halide exchange with chlorinated solvents as demonstrated by 31P{1H} NMR investigations on [MoBr(dppe)(η7-C7H7)].Syntheses of [FeI(dppe)Cp] from [FeI(CO)2Cp] and [MoX(dppe)(η7-C7H7)] (X = I, Br, Cl) from [MoX(CO)2(η7-C7H7)] are reported; substitution reactions of [MoX(CO)2(η7-C7H7)] with dppe proceed by a combination of associative and dissociative pathways.

The cycloheptatrienyl molybdenum alkynyl complex [Mo(C≡CH)(dppe)(η-C7H7)], 1 (dppe = Ph2PCH2CH2PPh2), undergoes oxidative dimerization on reaction with [FeCp2]PF6 in thf at −78 °C to give the bis(vinylidene) [{Mo(dppe)(η-C7H7)}2(μ-C═CH-CH═C)][PF6]2, [2][PF6]2. Deprotonation of [2][PF6]2 with KOBut yields butadiyndiyl-bridged [{Mo(dppe)(η-C7H7)}2(μ-C≡C-C≡C)], 3, which undergoes in situ aerial oxidation to give [{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6], [3]PF6, as the major product. The cyclic voltammogram of [3]PF6 exhibits a series of four redox processes indicative of sequential formation of [{Mo(dppe)(η-C7H7)}2(μ-C4)]n+ (n = 0, 1, 2, 3, 4) with the comproportionation constant, KC, for [3]PF6 of 1.9 × 107. Spectroscopic investigations on [3]PF6 by IR, Raman, NIR, and EPR spectroscopy reveal properties characteristic of a d5/d6 mixed valence complex with a localized electronic structure and an estimated intramolecular electron transfer rate in the range 108–1010 s–1. The experimental NIR spectrum of [3]PF6 is consistent with the predicted spectral characteristics of a three-state model for bridge-mediated, electron transfer in a weakly coupled, symmetrical mixed valence system. The dication [3][PF6]2 was isolated by chemical oxidation and structurally characterized; magnetic susceptibility measurements on [3][PF6]2 in the temperature range 2–300 K reveal strong antiferromagnetic coupling with the exchange coupling constant Jab (defined according to the Hamiltonian Ĥspin = –Jab·Ŝa·Ŝb) determined as −406 (±3) cm–1.

Two series of extended carbon chain butadiynyl and hexatriynyl complexes, [Mo{(C≡C)nC≡CSiMe3}(bpy)(η-C7H7)] (n = 1, 2; bpy = 2,2′-bipyridine) and [Mo{(C≡C)nC≡CR}(dppe)(η-C7H7)] (n = 1, R = H, SiMe3; n = 2, R = SiMe3; dppe = Ph2PCH2CH2PPh2), have been prepared and structurally characterized. The redox chemistry of these complexes has been investigated by cyclic voltammetry, and the 17-electron radical cations resulting from one-electron oxidation have been characterized by spectroelectrochemical IR and UV–visible methods and EPR spectroscopy. DFT calculations on the H-terminated model complexes [Mo{(C≡C)nC≡CH}(L2)(η-C7H7)]z+ (L2 = bpy, dppe) reveal a largely metal-centered HOMO (z = 0) with a modest increase in carbon chain character with increasing chain length. Spin density calculations for the 17-electron radical cations (z = 1) show large coefficients of spin density at the metal center, consistent with the remarkably high stability of the experimental complexes. However, both DFT theoretical and experimental synthetic studies highlight a distinction between the bpy- and dppe-supported systems. The 17-electron complexes [Mo{(C≡C)nC≡CSiMe3}(bpy)(η-C7H7)]PF6 (n = 1, 2) are unique examples of isolable, metal-stabilized butadiynyl and hexatriynyl radicals. In contrast, the dppe radical [Mo(C≡CC≡CSiMe3)(dppe)(η-C7H7)]+ exhibits chain-centered reactivity, consistent with enhanced coefficients of spin density at Cβ and Cδ in the model complex [Mo(C≡CC≡CH)(dppe)(η-C7H7)]+.

Two series of bis-phosphine-substituted cyclopentadienyl molybdenum alkynyl complexes, [Mo(C≡CR)(CO)(dppe)Cp′] and trans-[Mo(C≡CR)(CO)(PMe3)2Cp′] (R = Ph or C6H4-4-Me, dppe = Ph2PCH2CH2PPh2, Cp′ = Cp or Cp*), have been prepared and structurally characterized. One-electron oxidation to the 17-electron radical cations has been investigated by cyclic voltammetry and, for selected Cp* derivatives, by spectroelectrochemical IR and UV–visible methods. Through a combination of experimental measurements (IR and EPR spectroscopy) and DFT-based calculations some important differences between the two series of complexes [Mo(C≡CR)(CO)(dppe)Cp′] and trans-[Mo(C≡CR)(CO)(PMe3)2Cp′] have been established. In particular, the change in molecular geometry leads to enhanced alkynyl character in the HOMO of [Mo(C≡CR)(CO)(dppe)Cp′] when compared with the largely metal-centered HOMO of trans-[Mo(C≡CR)(CO)(PMe3)2Cp′].

Reaction of [FeCl(dppe)Cp*] with [Mo(C≡CC6H4-4-C≡CH)(dppe)(η-C7H7)], 1, and NaBPh4 in methanol gives the alkynylvinylidene complex [{Fe(dppe)Cp*}{μ-C≡CC6H4(H)C═C}{Mo(dppe)(η-C7H7)}]BPh4, [2A]BPh4, which is deprotonated to form the heterobimetallic, 1,4-diethynylbenzene-bridged complex [{Fe(dppe)Cp*}(μ-C≡CC6H4C≡C){Mo(dppe)(η-C7H7)}], 3. The alkynylvinylidene compound [2A]BPh4 exists as the major component of an equilibrium mixture with [Fe(dppe)Cp*}{μ-C═C(H)C6H4C≡C}{Mo(dppe)(η-C7H7)}]BPh4, [2B]BPh4, and is formed as a consequence of proton migration between the Cβ carbons of the vinylidene and alkynyl ends of the bridging ligand. Cyclic voltammetric investigations reveal that 3 undergoes two reversible one-electron oxidations to cationic [3]+ and [3]2+, which have been isolated as the [PF6]− salts after chemical oxidation. Computational (DFT) studies on [3]n+ indicate that while the HOMO of neutral 3 is rather heavily localized on the Mo center, in [3]+ the frontier orbitals are more evenly distributed over both metals, with the concentration of spin density being sensitive to the relative disposition of the metal end-cap fragments about the diethynylbenzene ligand. Experimental investigations on [3]PF6 by IR and EPR spectroscopy provide evidence for the coexistence of redox isomers [3A]+ and [3B]+, in which spin density is localized at the molybdenum or iron center, respectively. The solution IR spectrum of [3]PF6 exhibits an unusual four-band pattern in the ν(C≡C) region, consistent with the observation of two isomeric forms of [3]PF6, which are “valence trapped” on the short time scale of IR spectroscopy. In the frozen solution EPR spectrum, at 120 K, the spectroscopic signatures of both paramagnetic end-caps Mo(dppe)(η-C7H7) and Fe(dppe)Cp* are observed. The properties of [3]n+ are discussed with reference to the parent homobimetallics [{MLx}2(μ-C≡CC6H4C≡C)]n+ [MLx = Fe(dppe)Cp*, [4]n+, and Mo(dppe)(η-C7H7), [5]n+, n = 0, 1, 2].

The design and study of organometallic mixed-valence complexes is complicated by the mixing of metal d and bridging ligand π orbitals, which often makes the assignment of metal oxidation states ambiguous. However, in the case of complexes based on the cycloheptatrienyl-ligated molybdenum fragment, Mo(dppe)(η-C7H7), the strong ring to metal π bonding and metal to ring δ back-bonding interactions stabilize four of the metal d orbitals, while the dz2 orbital is destabilized by filled−filled interactions with the a-type MO of the C7H7 ring. When the Mo(dppe)(η-C7H7) auxiliary is used in conjunction with a 1,12-bis(ethynyl)-1,12-carbaborane-based bridging ligand, a weakly coupled (Robin and Day class II) mixed-valence system, [{Mo(dppe)(η-C7H7)}2{μ-1,12-(C≡C)2-1,12-C2B10H10}]+ ([2]+), with well-defined molybdenum oxidation states can be prepared. The near-IR region of [2]+ exhibits three intervalence charge transfer (IVCT) transitions and two lower intensity interconfigurational (or dd) transitions, which have been resolved through spectral deconvolution. The band shape of the lowest energy IVCT transition associated with [2]+, which arises from electron exchange between the dz2-type orbitals at the two Mo centers, is in excellent agreement with the predictions of the Hush two-state model for weakly coupled mixed-valence complexes. The half-height bandwidths of the higher energy IVCT transitions, which arise from transitions between lower lying metal orbitals that have symmetry properties that permit more significant mixing with the bridging ligand, are in less good agreement with the Hush model, due to the breakdown of the two-state approximation through the greater involvement of the bridge-based orbitals in those transitions.

A series of molybdenum alkynyl complexes [Mo(C≡CR)(dppe)(η-C7H7)] featuring a range of alkynyl substituents R with varying electron-donating and -withdrawing properties have been prepared. Oxidation of representative members of the series to the corresponding 17-electron radical cations has been achieved through both chemical oxidation and in situ spectroelectrochemical methods. The largely metal-centered character of the HOMO in this class of compounds has been established through a combination of experimental measurements (IR, UV−vis−NIR, EPR spectroscopies) and DFT-based calculations and rationalized in terms of the stabilization of the metal dxy, dyz, dxz, and dx2-y2 through π- and δ-interactions with the C7H7 ring and concomitant destablization of the dz2 orbital.

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)xO)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 dz2 orbital, whereas the HOMO of the [Ru(dppe)Cp]+ fragment is based on a metal dxy 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 dz2-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.

The paramagnetic aryl-alkynyl complexes [Mo(CCAr)(dppe)(η-C7H7)]+ (dppe = Ph2PCH2CH2PPh2; Ar = C6H5, [1]+; C6D5, [2]+; C6H4-4-F, [3]+; C6H4-4-Me, [5]+) and [Mo(CCBut)(dppe)(η-C7H7)]+ [4]+, have been investigated in a combined EPR and ENDOR study. Direct experimental evidence for the delocalisation of unpaired spin density over the framework of an aryl-alkynyl ligand has been obtained. The X-band solution EPR spectrum of the 4-fluoro derivative, [3]+, exhibits resolved hyperfine coupling to the remote para position of the aryl group [aiso(19F) = 4.5 MHz, (1.6 G)] in addition to couplings attributable to 95/97Mo, 31P and 1H of the C7H7 ring. A full analysis of the 1H ENDOR spectra is restricted by the low g anisotropy of the system which prevents the use of orientation selection. However, inter-comparison of the 1H cw-ENDOR frozen solution spectra of [1]+, [2]+, [4]+ and [5]+, combined with spectral simulation informed by calculated values derived from DFT investigations, has facilitated estimation of the experimental aiso(1H) hyperfine couplings of [1]+ including the ortho, ±3.7 MHz (±1.3 G) and para, ±3.9 MHz (±1.4 G) positions of the C6H5 substituent of the aryl-alkynyl ligand.