Electrospray-friendly ligands for the mass spectral analysis of transition-metal complexes
The key aim of this project was to develop the concept of electrospray-friendly ligands as a new ionisation method for the analysis of neutral metal complexes by electrospray mass spectrometry (ESMS). This involved introducing active functional groups into traditional ligands which do not otherwise undergo chemical ionisation under ESMS conditions. Electrospray-friendly ligands (as illustrated above for PPh₃ derivatives) were incorporated into a series of metal carbonyl complexes of Mo, Fe and Ru, metal halide complexes of Pd, Pt and Au, and a number of zero-valent complexes of Pt and Pd. All carbonyl complexes with ES-friendly phosphine, arsine and stibine ligands ionised by protonation, yielding strong [M + H]⁺ ions as the only signals in the spectrum. The metal halide complexes followed the known halide-loss mechanism. Pt(0) and Pd(0) complexes did not require functionalised ligands, as their PPh₃ derivatives already gave [M +H]⁺ ions. Mo and W carbonyl complexes of the two ‘naturally’ electrospray friendly ligands tpa (tpa = 1,3,5-triaza-7-phosphaadamantane) and tcep [tcep = tris(2-cyanoethyl)phosphine] could also be studied readily by ESMS. While all tpa complexes gave [M + H]⁺ ions, complexes of tcep preferred ionisation by NH₄⁺. The ionisation efficiencies of selected ligands and complexes are discussed. Thiolate complexes of Ni, Pd, Pt, Au and Hg were prepared using the ligands SC₆H₄OMe-p (S*) and p-SC₅H₄N (S•), but only the S•-complexes reliably underwent the protonation-type mechanism. The Cd-thiolate complexes [Cd₄(SPh)₁₀]²⁻, [S₄Cd₁₀(SPh)₁₆]⁴⁻ and [S₄Cd₁₀(SPh)₂₈]²⁻ underwent rapid ligand exchange with S*, but yielded insoluble products with S•. The complexes [Pt₂(μ-S)₂(L)₂] (L = PPh₃, P*, P**, P***. P•. As***¹) were prepared, and by means of ESMS, aspects of ligand exchange as well as reactivity towards other metal centres were investigated. Isonitrile derivatives of [Fe₃(CO)₁₂] and [Ru₃CO)₁₂] were prepared by ligand exchange with CNPh, CNC₆H₄OMe-p (CNPh*) and tosylmethylisocyanide. Substitution was observed for up to six CO ligands. Pyrolysis of [Fe₃(CO)₁₁(L)] and [Fe₃(CO)₁₀(L)₂] (L =CNPh, CNPh*) led to the products [Fe₃(CO)₉(μ₃ - η²-L)] and [Fe₃(CO)₈(μ₃-η²-L) (L)], respectively. All complexes could be studied by ESMS as their [M + H]⁺, [M + MeO]⁻ or [M – H]⁻ ions, depending on the exact nature of the analyte. [Fe₃(CO)₁₀(CNPh)₂] was characterised structurally (below) and revealed an unprecedented substitution pattern, together with unexpected partial disorder in the metal framework. Detection by ESMS of products of the type [Ru₄(CO) ₁₄₋ₙ(L)ₙ] (L = CNPh, CNPh*; n =2-4) in the reactions with ruthenium carbonyl led to the isolation and structural characterisation of [Ru₄(CO)₁₂(CNPh)₄], which is the only fully characterised isonitrile derivative of [Ru₃(CO)₁₂] with four metal atoms and exhibits an unusual imine-type bonding mode for the bridging isonitrile ligands. ________________________ ¹As*** = As(C₆H₄OMe-p)₃
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