Andrzej Wlodarczyk

Electrochemical and EPR spectral studies of mono- and bimetallic (trispyrazolylborato)molybdenum and tungsten complexes with extended di-phenol bridging ligands; evidence for electron exchange below the fast limit

Abstract Mononuclear diphenolato complexes [M(NO)L*X(OQOH)] and binuclear complexes [{M(NOL)L*X}2(μ-OQO)] {M = Mo, W; L* = [HB(3,5-Me2C3HN2)3]−; X = Cl, I; Q = C6H4, 4,4′-C6H4C(O)C6H4, 4,4′-C6H4S(O)2C6H4 or C6H4C(O)(C6H4)2C(O)C6H4} were prepared. Some have been reported before; the new complexes were thoroughly characterized. Electrochemical interactions between the metal centers in the binuclear complex with the three longer bridging ligands are small. The binuclear molybdenum complexes (which contain two 16-electron metal centres) with 4,4′-OC6H4C(O)C6H4O or 4,4′-OC6H4S(O)2C6H4O as bridging ligands were reduced to dianionic species (two 17-electron centres), whose EPR spectra indicate pairwise electron exchange at or near the fast limit; with the longer bridging ligand OC6H4C(O)(C6H4)2C(O)C6H4O the more complex EPR spectrum of the reduced dianionic complex indicates that electron exchange is no longer “fast”, but occurs at an intermediate rate. With the shortest bridging ligand OC6H4O the electrochemical interaction is sufficiently large to allow selective preparation of the mixed-valence 16-electron/17-electron species, whose EPR spectrum indicates that it is valence trapped; fast exchange is only initiated when both metal centres have a 17-electron configuration. The EPR spectra of the corresponding bimetallic tungsten dianions cannot be used to examine electron exchange processes since they show no resolved hyperfine signals.

DINUCLEAR MOLYBDENUM COMPLEXES DERIVED FROM DIPHENOLS : ELECTROCHEMICAL INTERACTIONS AND REDUCED SPECIES

Abstract The new diphenolato complexes [{Mo(NO){HB(dmpz)3}Cl}2Q] where dmpz = 3,5-dimethylpyrazolyl and Q = OC6H4(C6H4O (n = 1 or 2), OC6H4CRCRC6H4O (R = H or Et), and OC6H4CHCHC6H4CHCHC6H4O have been prepared and their electrochemical properties (cyclic and differential pulse voltammetry) compared with previously reported analogues where Q = OC6H4O, OC6H4EC6H4O (E = SO2, CO and S), OC6H4 (CO)C6H4 C6H4(CO)C6H4O and 1,5- and 2,7-O2C10H6. The electrochemical interaction between the redox centres in the new complexes is very weak, in contrast to that in the 1,4-benzenediolato and naphthalendiolato species. The EPR spectra of the reduced mixed-valence species [{Mo(NO){HB(dmpz)3}Cl}2Q]− where Q = 1,3- and 1,4-OC6H4O and OC6H4SC6H4O shows that they are valence-trapped at room temperature, whereas those of the dianions [{Mo(NO){HB(dmpz)3}Cl}2Q]2− where Q = 1,4-OC6H4O, OC6H4EC6H4O (E = CO or S) and OC6H4CHCHC6H4CHCHC6H4O shows that the unpaired spins on each molybdenum centre are strongly correlated (J, the spin exchange integral ⪢AMo, the metal-hyperfine coupling constant). The electrochemical properties and the comproportionation constants for the reaction [{Mo(NO){HB(dmpz)3} Cl}2Q] + [{ Mo ( NO ){ HB ( dmpz ) 3 } Cl } O ] 2 ] 2− ↩2[{ Mo ( NO ) { HB ( dmpz ) 3 } Cl } 2 Q ] − where Q = diphenolato bridge, are compared with related compounds containing benzenediamido and dianilido bridges.

Syntheses of 4-benzyl-3,5-dimethylpyrazolylborato complexes of molybdenum and tungsten nitrosyls: molecular structure of [Mo(CO)2(NO){HB(3,5-Me2-4-PhCH2C3N2)3}], a complex with an ‘inverted’ bowl-like structure

Reaction of [HB(bdmpz)3]–, obtained by reaction of 4-benzyl-3,5-dimethylpyrazole (Hbdmpz) with KBH4, with [M(CO)6](M = Mo or W) and p-MeC6H4SO2N(NO)Me afforded [M(CO)2(NO)-{HB(bdmpz)3}](M = Mo or W). The molybdenum carbonyl was converted into [Mo(NO)-{HB(bdmpz)3}X(Y)](X = Y = Cl, I or OC6H4Me-p; X = Cl, Y = OMe or OC6H4Me-p) the spectral and electrochemical properties of which are virtually identical to those of [Mo(NO){HB(dmpz)3}X(Y)](Hdmpz = 3,5-dimethylpyrazole), indicating that the addition of a benzyl group has no significant electronic effect in the new species. However, a crystal and molecular structure determination of [Mo(CO)2(NO){HB(bdmpz)3}] revealed that the molecule has a highly symmetrical structure, the CH2groups of the benzyl groups in the tris(pyrazolyl)borate ligand being disposed in a star-shaped configuration where the phenyl rings adopt an ‘inverted’ bowl-like structure with the BH group inside and the Mo(CO)2(NO) group outside the bowl. The arrangement of the 4-benzyl groups with respect to the pyrazolyl rings and the H–B ⋯ Mo three-fold axis is strongly reminiscent of an inverted three-legged ‘Isle of Man’ symbol.

Optically resolved 1-methylbenzylamido complexes of molybdenum and tungsten, and the crystal and molecular structure of s-iodo{tris (3,5-dimethylpyrazolyl)borato} {(+)-1-phenylethylamido}nitrosylmolybdenum, s-[Mo(NO){HB(3,5-Me2C3HN2)3} I{(+)-NHCH(Me)Ph}]

Abstract [M(NO)L*X2][M = Mo or W;L* = hydridotris(3,5-dimethylpyrazolyl)borate; X = Cl (Mo, W), X = I (Mo)] reacted with (+)- and (−)-1-phenylethylamine, NH2CH (Me)Ph, affording diastereomeric mixtures of [M(NO)L* X(NHCHMePh)]. These were separated by chromatography, and the structure of the pure diastereomer obtained by reaction of [Mo(NO)L*I2] with (+)-NH2CHMePh was determined crystallographically. The absolute configuration of this compound, [Mo(NO)L* I{(+)-NHCHMePh}], was determined to be S.

Complexes of tris(3,5-dimethylpyrazolyl)borates alkylated on the 4-position of the pyrazolyl rings. X-ray crystal structure of molybdenum dicarbonyl nitrosyl tris(3,5-dimethyl-4-n-butylpyrazolyl)borate

Abstract The 4-substituted tris(3,5-dimethylpyrazolyl)borato compounds [Mo(CO)2 (NO){HB(4-R-dmpz)3}] (R  Me, Et, Bun and C5H11) and [MoCl2(NO){HB(4-R-dmpz)3}] (R  Me or Bun) have been prepared and characterized spectroscopically and electrochemically. X-ray crystal structural determination of [Mo(CO)2(NO){HB(4-R-dmpz)3}] (R  Bun) revealed that the n-butyl chains attached to the 4-position of the pyrazolyl rings were oriented, overall, in a random fashion, although the first and second C atoms of each chain extended in a star-shaped configuration, similar to that observed in [Mo(CO)2(NO){HB(4-bzdmpz)3}] (4-bzdmpz = 4-benzyl-3,5-dimethylpyrazolyl).

Electrochemical and electron–electron exchange interactions in binuclear molybdenum complexes containing the bridging ligand 4-(imidazol-1-yl)phenol

A series of mono- and bi-nuclear complexes containing {M(NO)LX}[M = Mo or W, X = Cl or I, L = tris(3,5-dimethylpyrazol-1-yl)hydroborate] groups attached to either or both ends of the potentially bridging ligand 4-(imidazol-1-yl)phenol (HL1) have been prepared, in order to examine both electrochemical and electron–electron exchange interactions across the bridging ligand. Electrochemical studies have shown that the imidazolyl ligand shifts the redox potentials of the {Mo(NO)LX}(X = Cl or I) core cathodically to the extent that the 17e–18e reductions cannot be observed. As with the similar bridging ligand 4-(4-pyridyl)phenol, the electrochemical interaction between the two 16e–17e processes in the binuclear complex is weak. The binuclear molybdenum complexes of L1 contain one 17-electron (imidazolyl terminus) and one 16-electron (phenolate terminus) metal centre, and are clearly valence-trapped according to the EPR spectra. However one-electron reduction to the 17e–17e biradical initiates a strong electron–electron exchange interaction, whereby both of the unpaired electrons are coupled equally to both molybdenum nuclei, despite the weak electrochemical interaction.

New paramagnetic pyrazole complexes of tris(3,5-dimethylpyrazolyl)borato molybdenum nitrosyl chloride and iodide and the structure of [Mo(NO){HB(3,5-Me 2 C 3 HN 2 ) 3 }Cl(3,5-Me 2 C 3 HN 2 H)]

Abstract Reduction of [Mo(NO)L*Cl2][L* = HB(3,5-Me2C3HN2)3] with n-butyl lithium in the presence of SnBun3Cl afforded the paramagnetic (17-electron) [Mo(NO)L*Cl(3,5-Me2C3HN2H)] (1). Related compounds, [Mo(NO)L*XQ] (Q = pyrazole, 3,5-dimethyl-pyrazole, 4-bromopyrazole and 4-bromo-3,5-dimethylpyrazole; X = Cl or I), were prepared by the reduction of [Mo(NO)L*X2] by Q (X = I) or by n-butyl lithium in the presence of Q (X = Cl). IR and EPR spectroscopic characterization of these species is reported and the X-ray structure of [Mo(NO)L*Cl(3,5-Me2C3HN2H)]· 1 2 CH2Cl2, in which the molybdenum atom has octahedral coordination with disordered Cl and NO ligands, is described.

Alkoxy, thiophenolato and amido complexes of molybdenum tris-(3,5-dimethylpyrazolyl)borato nitrosyl acetate and benzoate, and the X-ray crystal structure of Mo{HB(3,5-Me2C3HN2)3}(NO)(NHPh)(OCOMe)·2CHCl3

Abstract The complexes MoL*(NO)(OR)(OCOR′) (L* = HB(3,5-Me 2 C 3 HN 2 ) 3 ; R = Me, Et, Pr n , Pr i ; R′= Me, Ph: R = CH 2 CH 2 OH, OH; R′= Ph: R = Ph, C 6 H 4 OMe- p ; R′= Me, Ph: R = C 6 H 4 Me-p; R′= Me), MoL*(NO)(SPh)(OCOR′) (R = Me or Ph) and MoL*(NO)(NHR)(OCOR′) (R = Me, Et, Ph; R′ = Me, Ph) were prepared by reaction of MoL*(NO)I(OR), MoL*(NO)I(SPh) and MoL*(NO)I(NHR) with AgOCOMe or AgOCOPh. The IR spectra of these complexes indicated that the carboxylate groups are monodentate and this was confirmed by a single crystal X-ray structural determination of MoL*(NO)(NHPh)(OCOMe).

Reaction of di-iodonitrosyl[tris(3,5-dimethylpyrazolyl)borato]-molybdenum with α,ω-alkanediols, 2-chloro-, and 2-methoxy-ethanol

The complexes [Mo{HB(3,5-Me2C3HN2)3}(NO)l{O(CH2)nR}][R = OH, n= 2–6; R = Cl or OMe, n= 2] and [Mo{HB(3,5-Me2C3HN2)3}(NO){O(CH2)nOH}2](n= 2) have been prepared by the reaction of [Mo{HB(3,5-Me2C3HN2)3}(NO)l2] with the appropriate α,ω-alkanediol, 2-chloro- or 2-methoxy-ethanol. No evidence could be found for chelated complexes of the type [[graphic omitted]}].

Reactions of ketones with molybdenum nitrosyl complexes: X-ray crystal structures of [{Mo{HB(3,5-Me2C3HN2)3}(NO)I}2O], a complex having a bent Mo–O–Mo bond, and of a bicyclic salt, [C6H3Me5N2(OH)][I1/2(I3)1/2]

Reaction of [Mo{HB(3,5-Me2C3HN2)3}(NO)I2] WITH refluxing acetone or diacetone alcohol led to C–C bond cleavage and formation, in low yield, of [{Mo {HB-(3,5-Me2C3HN2)3}(NO)I}2(µ–O)] and the bicyclic cation [C6H3Me5N2(OH)]+(isolated as the I– or mixed I–/I–3 salt); the structures of the last two were determined crystallographically, the µ-oxo species having a slightly bent Mo–O–Mo bond.