Dihydrobis(pyrazolyl)borate Alkylidyne Complexes of Tungsten

Poly(imidazolyliden-yl)borato Complexes of Tungsten: Mapping Steric vs. Electronic Features of Facially Coordinating Ligands

A convenient synthesis of [HB(HImMe)3](PF6)2 (ImMe = N-methylimidazolyl) is decribed. This salt serves in situ as a precursor to the tris(imidazolylidenyl)borate Li[HB(ImMe)3] pro-ligand upon deprotonation with nBuLi. Reaction with [W(≡CC6H4Me-4)(CO)2(pic)2(Br)] (pic = 4-picoline) affords the carbyne complex [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}]. Interrogation of experimental and computational data for this compound allow a ranking of familiar tripodal and facially coordinating ligands according to steric (percentage buried volume) and electronic (νCO) properties. The reaction of [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}] with [AuCl(SMe2)] affords the heterobimetallic semi-bridging carbyne complex [WAu(μ-CC6H4Me-4)(CO)2(Cl){HB(ImMe)3}].

Migratory insertion of isocyanide into a ketenyl-tungsten bond as key step in cyclization reactions

Treatment of the side-on tungsten alkyne complex of ethinylethyl ether [Tp*W(CO)2(η2-C,C'-HCCOCH2CH3)]+ {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)borate} (2a) with n-Bu4NI afforded the end-on ketenyl complex [Tp*W(CO)2(κ1-HCCO)] (4a). This formal 16 ve complex bearing the prototype of a ketenyl ligand is surprisingly stable and converts only under activation by UV light or heat to form a dinuclear complex [Tp*2W2(CO)4(μ-CCH2)] (6). The ketenyl ligand in complex 4a underwent a metal template controlled cyclization reaction upon addition of isocyanides. The oxametallacycles [Tp*W(CO)2{κ2-C,O-C(NHXy)C(H)C(Nu)O}] {Nu = OMe (7), OEt (8), N(i-Pr)2 (9), OH (10), O1/2 (11)} were formed by coordination of Xy-NC (Xy = 2,6-dimethylphenyl) at 4a and subsequent migratory insertion (MI) into the W-ketenyl bond. The resulting intermediate is susceptible to addition reactions with protic nucleophiles. Compounds 2a-PF6, 4a/b, and 7-11 were fully characterized including XRD analysis. The cyclization mechanism has been confirmed both experimentally and by DFT calculations. In cyclic voltammetry, complexes 7-9 are characterized by a reversible W(ii)/W(iii) redox process. The dinuclear complex 11 however shows two separated redox events. Based on cyclic voltammetry measurements with different conducting electrolytes and IR spectroelectrochemical (SEC) measurements the W(ii)/W(iii) mixed valent complex 11+ is assigned to class II in terms of the Robin-Day classification.

Synthesis, structure, and reactivity of a pyridine-stabilized silanonetungsten complex

A pyridine-stabilized silanonetungsten complex Cp*(OC)₂W{O[double bond, length as m-dash]SiMes₂(py)}(SiMe₃) (1b, Cp* = η⁵-C₅Me₅, Mes = 2,4,6-Me₃C₆H₂, py = C₅H₅N) was obtained by the reaction of a silyl(silylene) complex Cp*(OC)₂W([double bond, length as m-dash]SiMes₂)(SiMe₃) (3) with pyridine-N-oxide in pyridine. X-ray crystal structure determination revealed that complex 1b shows a similar geometry to that observed for a previously synthesized DMAP-stabilized analogue, Cp*(OC)₂W{O[double bond, length as m-dash]SiMes₂(DMAP)}(SiMe₃) (1a, DMAP = 4-NMe₂C₆H₄N). The Si[double bond, length as m-dash]O and W-O bond distances in 1b are comparable to those observed in 1a, but the nitrogen to silicon coordination bond of 1b is slightly longer (ca. 0.05 Å) than that of 1a, indicating the weaker coordination of pyridine than that of DMAP. The reaction of 1b with excess PMe₃ in C₆D₆ at r. t. proceeded via elimination of pyridine to afford a five-membered metallacyclic carbene complex, Cp*(OC)W([double bond, length as m-dash]C(SiMe₃)OSiMes₂O)(PMe₃) (5), but that of 1a with PMe₃ did not proceed at all. Complex 5 was further transformed in C₇D₈ at 100 °C for 4 h to give a four-membered W-O-Si-O metallacyclic complex with carbyne and PMe₃ ligands, Cp*W(OSiMes₂O)([triple bond, length as m-dash]CSiMe₃)(PMe₃) (7). The structural features of complexes 1b, 5, and 7 are comparable to those suggested theoretically as intermediates in the reaction of 3 with a sulfuration reagent to afford a six-membered metallacyclic carbene complex, Cp*W(S){[double bond, length as m-dash]C(SiMe₃)C([double bond, length as m-dash]O)OSiMes₂S} (6), indicating that complex 1b and the theoretically proposed silanethione complex are transformed via a similar reaction pathway.

Effect of ligand methylation on the spin-switching properties of surface-supported spin-crossover molecules

X-ray absorption spectroscopy investigations of the spin-state switching of spin-crossover (SCO) complexes adsorbed on a highly-oriented pyrolytic graphite (HOPG) surface have shown so far that HOPG is a promising candidate to realize applications such as spintronic devices because of the stability of SCO complexes on HOPG and the possibility of highly efficient thermal and light-induced spin-state switching. Herein, we present the spin switching of several Fe(II) SCO complexes adsorbed on an HOPG surface with particular emphasis on the thermally induced spin transition behaviour with respect to different structural modifications. The complexes of the type [Fe(bpz)₂(L)] (bpz  =  dihydrobis(pyrazolyl)borate, L  =  1,10-phenanthroline, 2,2'-bipyridine) and their methylated derivatives exhibit SCO in the solid state with some differences regarding cooperative effects. However, in the vacuum-deposited thick films on quartz, complete and more gradual spin transition behavior is observable via UV/vis spectroscopy. In contrast to that, all complexes show large differences upon direct contact with HOPG. Whereas the unmodified complexes show thermal and light-induced SCO, the addition of e.g. two or four methyl groups leads to a partial or a complete loss of the SCO on the surface. The angle-dependent measurement of the N K-edge compared to calculations indicates that the complete SCO and HS-locked molecules on the surface exhibit a similar preferential orientation, whereas complexes undergoing an incomplete SCO exhibit a random orientation on the surface. These results are discussed in the light of molecule-substrate interactions.

Relative hemilabilities of H2B(az)2 (az = pyrazolyl, dimethylpyrazolyl, methimazolyl) chelates in the complexes [M(η-C3H5)(CO)2{H2B(az)2}] (M = Mo, W)

The question of B–H–Mo hemilability in a range of dihydrobis(azolyl)borate scorpionate ligands is discussed with reference to η³-allyl complexes [Mo(η³-C₃H₅)(CO)₂{H₂B(az)₂}] [az = pyrazolyl (pz), dimethylpyrazolyl (pz*), mercaptoimidazolyl (mt)].

Synthesis and characterization of imidazolium poly(azolyl)borate ionic liquids and their potential application in SO2 absorption

New imidazolium poly(azolyl)borate ionic liquids (ILs) were synthesized by salt metathesis, and they can absorb SO₂ through multi-site interactions at 20 °C, 0.1 MPa SO₂ with the highest SO₂ capacity of 5.8 mol mol⁻¹ of IL and 1.05 g g⁻¹ of IL.