Stephan Kohlmann

What determines the solvatochromism of metal-to-ligand charge transfer transitions? A demonstration involving 17 tungsten carbonyl complexes

Abstract A consistent model which permits rationalization and estimation of the solvatochromic behaviour of coordination compounds with metal-to-ligand charge transfer absorption bands is described. The model shows how the changing relationship between metal-ligand bond polarities in the ground and MLCT excited state determines whether negative, positive, or no solvatochromism results. Data for seventeen mononuclear and binuclear tetra- and pentacarbonyltungsten complexes are analyzed in order to illustrate and substantiate different electronic situations leading to various degrees of solvatochromism. Ligand basicities, calculated Huckel molecular orbital coefficients, ESR coupling constants, and metal fragment oxidation potentials are used to estimate the ability of metal fragments and ligands for charge transfer in the excited state and the resulting solvatochromism of complexes.

Metal—metal interaction in ligand-bridged dimolybdenum(0,0) and -(I,0) complexes with very small frontier orbital gaps: electrochemistry and spectroscopic properties of three neighboring oxidation states☆

Complexes (μ-L)[Mo(CO)2(PnBu3)2]2 with cis-carbonyl ligands and symmetrically bridging bis(α-diimine) chelate ligands L = 2,2′-bipyrimidine (bpym) and 2,5-bis(2-pyridyl)pyrazine (2,5-bppz) were synthesized and studied in different oxidation states by cyclic voltammetry, 1H NMR, EPR, IR and UV-Vis-NIR absorption spectroscopy. The combination of two very electron rich d6 metal centers with one π accepting bridging ligand results in rather small HOMO-LUMO energy gaps, as evident from differences of about 1 V between the potentials for reversible oxidation and reduction and from intense charge transfer absorption at about 1 eV, i.e. in the near-infrared (NIR) region. Whereas the shifts of the carbonyl vibrational bands are about equal for the reduction (low-energy shift) and the oxidation (high-energy shift) of the more stable bpym complex, the EPR results reveal occupation of the α-diimineπ ∗ orbitals in the anionic forms and an Mo(I)/Mo(0) mixed-valent state with a comproportionation constant Kc greater than 108 for the cation {(bpym)[Mo(CO)2(PnBu3)2]2}+. Electro transitions observed by UV-Vis-IR spectroelectrochemistry confirm these assignments of the redox orbitals; the maximum of the weak metal-to-metal charge transfer band of the mixed-valent complex {(bpym)[Mo(CO)2(PnBu3)2]2}+ was found at 3700 mm

Spectroelectrochemistry of aromatic ligands and their derivatives: III. Binuclear transition metal complexes of CuI, Mo0, and ReI with 2,2′-bipyrimidine☆

The binuclear complexes [Mo(CO)4]2(bpym) (I), [Re(CO)3Cl]2(bpym) (II), and [[Cu(PPh3)2]2-(bpym)]2+ (III) were subjected to one- and (for I, III) two-electron reduction, and the products studied in situ by UVVisNIR spectroscopy. The spectra were assigned in terms of a simple Huckel molecular orbital scheme, in which the reduction orbital is ligand π(7), related to π(7) of biphenyl, the transition π(6) → π(7) moves to lower energy on successive reduction, and bands observed in the near IR-visible region are due to transitions from π(7) to higher unoccupied orbitals. Detailed assignments are directly related to those of other singly and doubly reduced azabiphenyls; the bpym dianion has been characterized for the first time.

First observation of resolved ESR hyperfine structure for a reduced ruthenium(II) polyazine complex

Abstract A resolved ESR spectrum of a ruthenium(II) polyazine radical complex has been observed for the first time in the binuclear species [(bpy)4Ru2(dptz)]3+ (dptz: 3,6-di-2-pyridyl-1,2,4,5-tetrazine). Electron spin resonance in conjunction with HMO McLachlan calculations reveal the localization of the unpaired electron in the tetrazine π system and a strongly polarizing effect of the [Ru(bpy)2]2+ fragment.

Mono- and binuclear molybdenum carbonyl complexes with charge-transfer absorptions in the near IR

Abstract Molybdenum(0) carbonyl complexes with very long wavelength charge-transfer absorptions (λ = 700–1000 nm) have been obtained via lowering of the complex, lowest unoccupied molecular orbital, viz. by using strongly π-accepting α-diimine ligands, and via raising of the metal highest occupied molecular orbital, viz. by employing the electron-rich metal fragment Mo(P n Bu 3 ) 2 (CO) 2 . The electronic structures of these compounds were studied by electron and IR spectroscopy, by cyclic voltammetry and by ESR of the anion radicals.

Reinvestigation of the visible absorption bands of the 2,2′-bipyrimidine complexes W(CO)4(bpym) and (μ-bpym)[M(CO)4]2 (M=Mo, W) with resonance Raman spectroscopy; the emission spectrum of (μ-bpym)[Mo(CO)4]2

Abstract The character of the two lowest energy transitions of W(CO) 4 (bpym) and (μ-bpym)[M(CO) 4 ] 2 (M=Mo, W) were established with resonance Raman spectroscopy. According to these spectra the two bands belong to MLCT transitions to different π* orbitals of the bpym ligand. Contrary to expectations it is not the first (lowest energy) but the second and more intense electronic transition which, according to the resonance Raman spectra, is directed to the lowest lying π* orbital (b 2u *, LUMO) of these complexes. This interpretation explains the different band intensities and the untypically low g values of the ESR signals of corresponding anion radicals. Excitation of (μ-bpym)[Mo(CO) 4 ] 2 in CH 2 Cl 2 at 400 nm produced a weak emission with an onset at 700 nm. According to the excitation spectrum, this emission originates from the lowest MLCT-excited state of the complex.

‘Inverse cryptate’ structure of an exceptionally stable dicopper(I) semiquinonoid intermediate

Dinuclear diphosphinecopper(I) complexes of the bis(chelating)‘S-frame’ ligand di-tert-butyl azodiformate exhibit a remarkable kinetic and thermodynamic stability of the deep blue o-semiquinonoid intermediate as evident from its facile formation, stability towards air and protic media, and from the electrochemical potential range. The comproportionation constant of [CuI2{µ-N2[CO(OBut)]2}{µ-Ph2P(CH2)6PPh2}2]+ was established at 1019·7. The crystal structure of the tetraphenylborate salt has been determined. It shows an ‘inverse cryptate’ structure; two bridging diphosphine ligands span the two bridgehead copper(I) centres which are fixed at 4.82 A apart by the bis(chelating) azodicarboxylate anion radical. In contrast to the neutral (reduced) form of the complex, the dicationic oxidised state could only be spectroelectrochemically.

cis-Dicarbonylbis(tributylphosphan)molybdän-Komplexe der vier isomeren Bidiazine

Abstract The complexes cis -Mo(CO) 2 (PBu 3 ) 2 (bdz) which have the four isomeric bidiazine (bdz) ligands 3,3′-bipyridazine, 2,2′-bipyrazine, 2,2′- and 4,4′-bipyrimidine, can undergo reversible one-electron oxidation and reduction and show small redox potential differences of less than 1.5 V. The small HOMOLUMO gap gives rise to long-wavelength metal-to-ligand charge transfer absorptions, an assignment which is supported by ESR studies. Guidelines for the construction of complexes with small charge-transfer absorption energies by CO/PR 3 exchange are presented. Although a ligand-centered MO is occupied during reduction, the small g factors of the radical complexes indicate low-lying ligand-field-excited states which are believed to be responsible for the pronounced light-sensitivity of the compounds.

A 95Mo NMR study of electronic effects in tetracarbonyl−molybdenum complexes of aromatic α-diimine chelate ligands

Abstract Studies of nine (α-diimine)MO(CO) 4 complexes demonstrate the importance of metal-to-ligand charge-transfer contributions for the chemical shift in 95 Mo NMR spectroscopy. The δ( 95 Mo) values, which occur in the range − 847 to − 1164 ppm, correlate with the calculated LUMO coefficients c N 2 at the coordinating (chelate) nitrogen centres, with the McLachlan π-spin populations p N ML and with ESR coupling constants a ( 14 N) of the anion radical complexes.

Electron Transfer Induced Metal Addition and Ligand Exchange in Organometallic Anion Radical Complexes

Transition metal complexes containing anion radical ligands display a strong tendency towards full coordinative saturation at the singly reduced ligand and towards substitutional activation of coligands at the metal center. Examples involving metal carbonyls show how both of these reactivities can be employed either separately or in a combined fashion for electron transfer catalyzed substitution processes and for the construction of new polynuclear complexes with unusual properties.