A Spectroscopic and Computationally Minimal Approach to the Analysis of Charge-Transfer Processes in Conformationally Fluxional Mixed-Valence and Heterobimetallic Complexes.

Class II mixed-valence bimetallic complexes {[Cp'(PP)M]C≡C-C≡N[M'(PP)'Cp']}2+ (M, M'=Ru, Fe; PP=dppe, (PPh3 )2 ; Cp'=Cp*, Cp) exist as conformational ensembles in fluid solution, with a population of structures ranging from cis- to trans-like geometries. Each conformer gives rise to its own series of low-energy intervalence charge-transfer (IVCT) and local d-d transitions, which overlap in the NIR region, giving complex band envelopes in the NIR absorption spectrum, which prevent any meaningful attempt at analysis of the band shape. However, DFT and time-dependent (TD)DFT calculations with dispersion-corrected global-hybrid (BLYP35-D3) or local hybrid (lh-SsirPW92-D3) functionals on a small number of optimised structures chosen to sample the ground state potential energy hypersurfaces of each of these complexes has proven sufficient to explain the major features of the electronic spectra. Although modest in terms of computational expense, this approach provides a more accurate description of the underlying molecular electronic structure than would be possible through analysis of the IVCT band by using the static point-charge model of Marcus-Hush theory and derivatives, or TDDFT calculations from a single (global) minimum energy geometry.

Single-Molecule Conductance Studies of Organometallic Complexes Bearing 3-Thienyl Contacting Groups.

The compounds and complexes 1,4-C6 H4 (C≡C-cyclo-3-C4 H3 S)2 (2), trans-[Pt(C≡C-cyclo-3-C4 H3 S)2 (PEt3 )2 ] (3), trans-[Ru(C≡C-cyclo-3-C4 H3 S)2 (dppe)2 ] (4; dppe=1,2-bis(diphenylphosphino)ethane) and trans-[Ru(C≡C-cyclo-3-C4 H3 S)2 {P(OEt)3 }4 ] (5) featuring the 3-thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3-5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break-junction (STM-BJ) and STM-I(s) methods (measuring the tunnelling current (I) as a function of distance (s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM-I(s) methods, and a higher feature by the STM-BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi-parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non-resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2-5.

Multifunctional switching properties of "wire-like" dinuclear ruthenium bis-alkynyl spiropyran complexes.

Multifunctional switches are crucial to the development of smart molecular materials and molecular-electronic applications. Here, we describe the synthesis, structure, and characterization of several spiropyrans functionalized with alkynyl-[Ru(dppe)2] moieties. Through electrochemical and spectroelectrochemical studies, we demonstrate access to several stable redox states, in addition to states accessed via acidochromism and photoisomerisation. Initial protonation was found to occur at the alkynyl functionality followed by acid-induced ring-opening of the spiropyran ligand to form the protonated merocyanine. The protonated merocyanine can be switched from the Z- to E-isomer by using ultraviolet light. The spiropyran was also shown to be an effective insulator for electronic communication across the molecular backbone.

Syntheses, structural characterisation and electronic structures of some simple acyclic amino carbene complexes.

Reactions of the vinylidene complexes [M([double bond, length as m-dash]C[double bond, length as m-dash]CH2)(PP)Cp']PF6 with NH3 readily afford the acyclic amino carbene complexes [M{C(Me)NH2}(PP)Cp']PF6 [M(PP)Cp' = Fe(dppe)Cp, Ru(dppe)Cp, Ru(PPh3)2Cp, Ru(dppe)Cp*]. Crystallographic, spectroscopic and computational data support a description of these complexes in terms of a M-{C(Me)[double bond, length as m-dash]N(+)H2} mesomeric form, and simple σ-coordination of the singlet acyclic amino carbene.

Reactions of alkynes with cis-RuCl2(dppm)2: exploring the interplay of vinylidene, alkynyl and η(3)-butenynyl complexes.

Reactions of cis-RuCl2(dppm)2 with various terminal alkynes, of the type HC≡CC6H4-4-R (1 equiv.), in the presence of TlBF4 have resulted in the formation of cationic vinylidene complexes trans-[RuCl(=C=CHC6H4-4-R)(dppm)2]BF4 ([1]BF4). These complexes can be isolated, or treated in situ with a suitable base (Proton Sponge, 1,8-bis(dimethylamino)naphthalene) to yield the mono-alkynyl complexes trans-RuCl(C≡CC6H4-4-R)(dppm)2 (2). Through similar reactions between cis-RuCl2(dppm)2 with 2 equiv. of alkyne, TlBF4 and base, trans-bis(alkynyl) complexes, trans-Ru(C≡CC6H4-4-R)2(dppm)2 (3), can be isolated when R is an electron withdrawing substituent (R = NO2, COOMe, C≡CSiMe3), whereas reactions with alkynes bearing electron donating substituents (R = OMe and Me) form cationic η(3)-butenynyl complexes [Ru(η(3)-{HC(C6H4-4-R)=CC≡CC6H4-4-R})(dppm)2](+) ([4](+)). This work highlights the importance of the electronic character of the alkyne in influencing product outcome.

Rapid Markovnikov addition of HCl to a pendant alkyne: evidence for a quinoidal cumulene.

Reaction of cis-[RuCl2(dppm)2]BF4 with TlBF4 and 1,4-diethynyl-benzenes results in the formation of the vinylidene cations trans-[Ru([double bond, length as m-dash]C[double bond, length as m-dash]CH-C6H2-2,5-R2-4-C[triple bond, length as m-dash]CH)Cl(dppm)2](+) (R = H, Me). Subsequent reaction with [N(n)Bu4]Cl results in nucleophilic attack at the coordinated organic ligand, but not at the expected metal-bound carbon atom. Instead, trans-[Ru(C[triple bond, length as m-dash]C-C6H2-2,5-R2-4-CCl[double bond, length as m-dash]CH2)Cl(dppm)2] was generated which, when coupled with DFT calculations, provides evidence for an intermediate quinoidal cumulene complex.

Long range charge transfer in trimetallic mixed-valence iron complexes mediated by redox non-innocent cyanoacetylide ligands.

The reaction of Fe(C≡CC≡N)(dppe)Cp (1) with one-half equivalent of [trans-Fe(N≡CMe)2(dppx)2][BF4]2 (dppx = dppe ([2][BF4]2) or dppm ([3][BF4]2)) affords trimetallic [trans-Fe{N≡CC≡CFe(dppe)Cp}2(dppx)2][BF4]2 (dppx = dppe [4][BF4]2; dppx = dppm [5][BF4]2). Both [4][BF4]2 and [5][BF4]2 undergo three, one-electron oxidation processes, arising from sequential oxidation of the two terminal Fe(C≡CR)(dppe)Cp moieties and finally the central Fe(N≡CR)2(dppx)2 fragment. The redox products [4](n+) and [5](n+) (n = 3, 4) have been characterised by UV-vis-NIR and IR spectroelectrochemistry. The shifts in the characteristic ν(C≡CC≡N) bands upon oxidation demonstrate not only the localised electronic structure of the trications, but also the redox non-innocence of the cyanoacetylide ligands. The trimetallic [formally Fe(II/II/III) mixed-valence] complexes [4](3+) and [5](3+) feature two distinct IVCT transitions, one associated with charge transfer from the central 18-electron {Fe(N≡CR)2(dppx)2}(2+) to terminal {Fe(C≡CR)(dppe)Cp}(+) moiety, and a lower energy transition involving charge transfer between the terminal Fe fragments which are separated by the redox active 9-atom, 10-bond -C≡C-C≡N{Fe(dppx)2}N≡C-C≡C- bridge. The tetracationic complexes [4](4+) and [5](4+) generated by a further stepwise oxidation exhibit a single {Fe(N≡CR)2(dppx)2}(2+)→{Fe(C≡CR)(dppe)Cp}(+) IVCT transition.

The preparation, characterisation and electronic structures of 2,4-pentadiynylnitrile (cyanobutadiynyl) complexes.

Convenient preparative routes to mononuclear ruthenium complexes containing the 2,4-pentadiynylnitrile, or cyanobutadiynyl, ligand are described. The electronic properties of the [C(5)N](-) ligand are closely related to those of not only the cyanide ([CN](-)) and 2-propynylnitrile or cyanoacetylide ([C≡CC≡N](-)) ligands, but also those of the isoelectronic polyynyl ([{C≡C}(n)R](-)) ligands.