Kazuo Kashiwabara

Structural and spectroscopic comparisons in two series of cobalt(III)–monodentate phosphine complexes: trans-[Co(acac)2(PMe3−nPhn)(H2O)]PF6 and trans-[Co(acac)2(PMe3−nPhn)2]PF6 (acac=pentane-2,4-dionate; n=0, 1, 2 and 3)

Abstract The X-ray crystal structures and spectroscopic properties of two series of cobalt(III)–monodentate phosphine complexes, trans-[Co(acac)2(PMe3−nPhn)(H2O)]PF6 (na) and trans-[Co(acac)2(PMe3−nPhn)2]PF6 (nb), have been compared in variation of methyl/phenyl substituents of phosphines. The CoP bond lengths in complexes nb are found to be remarkably longer (>0.095 A) than those in the corresponding complexes na. Complex trans-[Co(acac)2(PPh3)(H2O)]PF6 (3a) shows an unusually long CoP bond (2.243(1) A) and an anomalous distortion of the equatorial acac planes, owing to a severe steric interaction associated with three phenyl rings of PPh3. It appears that the PPh3 in 3a is bound more closely to cobalt(III) in solution than in the solid state, as suggested by comparison of the absorption spectra of the solution and the solid. The ligand spectral parameters of phosphines, dCo(PMe3−nPhn), are estimated by using the energy of the a1Eg←1A1g transition, which is observed as an absorption band for complexes na or a broad shoulder for complexes nb, and evaluated in relation to the 59Co NMR chemical shifts. The estimated dCo(PMe3−nPhn) values indicate that the ligand-field strengths of the phosphines become considerably reduced as the number of phenyl substituents increases. Also, the ligand-field strengths of the phosphines in complexes nb are smaller than those in the corresponding complexes na. Furthermore, the low-lying (18500–25000 cm−1) LMCT band of complexes nb can be explained by the estimated parameters.

SYNTHESES AND STRUCTURES OF RUTHENIUM(II) COMPLEXES BEARING HYBRID PHOSPHINE-THIOETHER LIGANDS, ME2PCH2CH2SR

Abstract Hybrid phosphine-thioether ligands Me 2 PCH 2 CH 2 SR (L) reacted with [RuCl 2 (cym)] 2 (cym= p -cymene) to produce 11 new ruthenium(II) complexes; [RuCl 2 (cym)(L)] ( 1 – 3 for R=CH 3 (Me), C 2 H 5 (Et) and C 6 H 5 (Ph), respectively) in toluene, [RuCl(cym)(L)] + ( 4 – 6 for R=Me, Et and Ph, respectively) in ethyl alcohol and [RuCl 2 (L) 2 ] ( 7 and 8 for R=Me and Et, respectively and 9 – 11 for R=Ph) in refluxing n -butanol. Complexes 1 – 3 in which L acts as a monodentate ligand through phosphorus led to complexes 4 – 6 where L binds to Ru through phosphorus and sulfur in ethyl alcohol at room temperature. Complexes 9 – 11 were separated into three of five possible geometrical isomers by fractional crystallization, trans (Cl,Cl′) trans (P,P′) ( 9 ), cis (Cl,Cl′) cis (P,P′) ( 10 ) and trans (Cl,Cl′) cis (P,P′) ( 11 ), whereas complexes 7 and 8 afforded only the trans (Cl,Cl′) trans (P,P′) isomer. The crystal structures of complexes 5 , 8 , 9 and 11 were determined by an X-ray diffraction method, suggesting stronger trans influence of the dimethylphosphino group than those of the thioether and p -cymene moieties. 31 P-{H}- and 1 H- or 13 C-{H}-NMR spectral data are used to characterize the structures of the complexes. The hydride complex [RuH(cym)(Me 2 PCH 2 CH 2 SEt)] + was also prepared by the treatment of NaBH 4 with complex 5 .

Syntheses and crystal structures of geometrical isomeric pairs: Trans- and cis-(PPh4) [Co (acac)2 (CN)2] and trans- and cis-[Co(acac)2(PMe3 or PEt3)2]PF6 (acac = pentane-2,4-dionate)

Abstract Reactions of trans -[Co(acac) 2 (PPh 4 ) 2 ]PF 6 (aca = pentane-2,4-dionate) with PR 3 (R = Me or Et) in methanol afforded new complexes, trans -(PPh 4 ) [Co(acac) 2 (CN) 2 ] and trans -[Co(acac) 2 (PR 3 ) 2 ]PF 6 , respectively. The structures of the new trans -isomers, together with those of the corresponding known cis -isomers, were determined by single-crystal X-ray diffraction. Complexes trans -[Co(acac) 2 (PR 3 ) 2 ] =− were stable in dry organic solvents, but in wet methanol easily hydrolysed to give trans -[Co(acac) 2 (PR 3 )(H 2 O)] − and isomerized to cis -[Co(acac) 2 (PR 3 ) 2 ] − were stable in both dry and wet solvents. The X-ray structure analyses showed that the CoC and CoP bonds in the trans -isomers were longer by about 0.07 and 0.08 A, respectively, than those of the corresponding cis -isomers. The CoO bonds trans to cyanide and phosphine ligands were also longer than those of the mutually trans CoO bonds, owing to the strong trans influences of cyanide and phosphine ligands.

Preparation and characterization of rhodium(III) complexes containing 1,1,1-tris(dimethylphosphinomethyl)ethane (tdmme). Structures of [RhX3(tdmme)](X = Cl, Br or I), [Rh(NH3)3(tdmme)]3+ and [{Rh(tdmme)}2(µ-X)3]3+(X = Cl or OH) in the solid state and in solution

Several new rhodium(III) complexes containing a tripodal tridentate phosphine, MeC(CH2PMe2)3(tdmme), have been synthesized and their structures investigated both in the solid state and in solution. Single-crystal X-ray analyses revealed the smaller steric requirement and stronger trans influence of tdmme than those of the phenyl-substituted analogue, MeC(CH2PPh2)3(tdpme). The long Rh–Cl bond in the mononuclear trichloro complex, [RhCl3(tdmme)]1, due to the strong trans influence of tdmme, made the complex reactive in water; the structure of 1 in solution and its reaction products with some acids or bases were characterized. The triply chloro-bridged dinuclear complex, [{Rh(tdmme)}2(µ-Cl)3][BF4]3, also showed structural change in water, while the mononuclear triammine complex, [Rh(NH3)3(tdmme)][BF4]3, and the triply hydroxo-bridged dinuclear complex, [{Rh(tdmme)}2(µ-OH)3][BF4]3, were stable. The absorption spectra of the tdmme complexes suggested a stronger ligand field of tdmme to the rhodium(III) centre than those of corresponding didentate diphosphine ligands, in accordance with the short Rh–P bond lengths in 1 found by single-crystal X-ray analysis.

Synthesis and structural characterization of cobalt(III) complexes of hybrid donor-type didentate or tetradentate ligands bearing dimethylphosphino and thioether groups

Abstract The preparation and characterization of new hybrid donor-type phosphine-thioether ligands, a linear tetradentate Me2P(CH2)2S(CH2)3S(CH2)2PMe2 (dmdtn) and a didentate Me2P(CH2)2SR [R = Me (mtdmp) or Et (etdmp)], and their cobalt(III) complexes are described. The reaction of trans-[CoCl2(PY)4]C1 (py = pyridine) with dmdtn, followed by anion metathesis with NaPF6, afforded green crystals of trans(Cl,Cl-[CoCl2{meso(S)-dmdtn}]PF6 (1), whose geometrical structure and configuration around S atoms were determined by X-ray analysis. Further reaction of 1 with Li(acac) (acac = pentane-2,4-dionate) and crystallization with LiBF4 gave red crystals of the acac complex 2. The X-ray analysis of 2 confirmed the structure and configuration of cisβ-[Co(acac){racemic(S)-dmdtni}(BF4)2, indicating the configurational inversion at one sulfur during the derivation of complex 2. An analogous N,N-dimethyldithiocarbamate (dtc) complex 3 was also prepared, and the electrochemical properties of complexes 1–3 were discussed. For analogous mtdmp and etdmp (P-S) complexes, trans(Cl,Cl)cis(P,P)-[CoCl2(P-S)2]PF6 reacted with Li(acac) to give not only trans(P,S)-[Co(acac)(P-S)2](PF6)2 but also [Co(acac)2(P-S)]PF6. Similar reactions with Na(dtc) yielded cis-[Co(dtc)2(P-S-κP)2] (14 for P-S = mtdmp), together with trans(P,S)-[Co(dtc)(P-S)2](PF6)2 and [Co(dtc)2(P-S)]PF6. The structures of these complexes were characterized by NMR and UV-visible spectroscopy and X-ray analysis of 14. These results suggested that the didentate ligands were hemilabile as similar to previously described SPPS-type tetradentate ligands, but the tetradentate dmdtn ligand was not.

Redox potentials of a series of bis(2,4-pentanedionato)cobalt(III) complexes containing amine, phosphine, arsine, or their hybrid donor didentate ligands. σ and π contributions in the CoIIIN, P and As bonding

Reduction (E12(red)) and oxidation potentials (E12(ox)) of [Co(acac)2(L)]+ (acac = 2,4-pentanedionate) complexes containing an NN, NN′, N′N′, NP, PP, NAs, or AsAs′ didentate ligand as L(N = −CH2NH2; N′ = −CH2NMe2; P = −CH2PMe2; As = −CH2AsMe2; As′ = −CH2CH2AsMe2) were determined by electrochemical measurements. The E12(red) values which reflect the dσ∗(Co) orbital (homo) energy shift negatively in the following order: (i) L = N′N′>N′N>NN; (ii) L = N′N > AsN > PN; and (iii) L = N′N′ > AsAs′ > PP. The E12(ox) values shift positively in the following order: L = PP, AsAs′ < PN, AsN < NN, N N′, N′N′. This order suggests that the dπ(Co) orbital is more destabilized by the phosphine or arsine ligands than the amine ones.

Cobalt(II) phosphine complexes stable in aqueous solution: spectroscopic and kinetic evidence for low-spin Co(II)P6 and Co(II)P3S3 with tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane

Abstract Co(II) species produced by the controlled potential electrochemical reduction of Co(III)P 3 S 3 and Co(III)P 6 complexes with 1,4,7-trithiacyclononane and tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane in aqueous solution were characterized at ambient temperature by the EPR and spectrophotometric methods: the Co(II)P 6 and Co(II)P 3 S 3 species are in the low-spin t 2g 6 e g 1 state with large Jahn–Teller distortion. Kinetic studies of the redox reactions involving these Co(III)/(II) species revealed that the electron self-exchange reactions for the Co(III)/(II) couples are very fast ( k ex ∼10 4 dm 3 mol −1 s −1 ), which is consistent with the results for other low-spin/low-spin Co(III)/(II) couples. It was concluded that the nephelauxetic effect of the P donor atom stabilizes the low-spin state in Co(II).

Synthesis, crystal structures and spectroscopic properties of cis- and trans-[RhCl2L2]+ [L = 1,3-bis(dimethylphosphino)propane or 1,2-bis(dimethylphosphino)ethane (dmpe)]: Reinvestigation of the dmpe complexes

Two pairs of geometrical isomers, cis- and trans-[RhCl2L2]+[L = 1,3-bis(dimethylphosphino)propane or 1,2-bis(dimethylphosphino)ethane], have been synthesized and their structures and spectroscopic properties investigated. The previous characterization of the dmpe complexes made by other workers was found to be erroneous. A convenient method of preparation and separation of the complexes into isomers has been found. The geometrical structures of the isomers have been confirmed by 1H, 13C and 31P NMR spectroscopy and by single-crystal X-ray structure determinations. The Rh–Cl bond lengths in the cis complexes are considerably longer than those in the corresponding trans isomers, and the Rh–P bonds trans to Cl in the cis isomers are relatively shorter than those of mutually trans phosphine ligands in both the cis- and trans-isomers, exhibiting a strong trans influence of the dimethylphosphino group. The 1H, 13C and 31P NMR spectra of the complexes were found to be consistent with the structures found by single-crystal X-ray analyses, and the chemical shifts (δp) and coupling constants [1J(RhP) and 2J(PP)] correspond well with the structural parameters (Rh–P bond length and P–Rh–P angle). The electronic spectra and isomerization reactions of the complexes were also determined.

Preparation, circular dichroism and substitution reactions of platinum(II) complexes with asymmetric olefin ligands

A review is presented of separation of optically active platinum(II) complexes with prochiral olefin ligands by use of the diastereoisomers formed with optically active aminocarboxylates, and of their CD spectra. The CD pattern shown is to be determined not only by the absolute configuration of olefin ligands but also by the asymmetric nitrogen formed upon coordination of the amino moiety. The change in CD on olefin substitution is useful for kinetic studies. The trans effect, stereoselectivity, and asymmetric induction which accompanies the substitution are discussed in terms of steric hindrance and mutual ligand interaction.

Preparation, crystal structures and isomerization kinetics of cis- and trans-[Co(dtc)2(PHPh2)2]+: thermodynamically and kinetically stable cobalt(III)–P bonds through interplay of σ-donicity, π-acidity, and steric bulkiness

Novel cobalt(III)–diphenylphosphine complexes, cis- and trans-[Co(dtc)2(PHPh2)2]+, were synthesized and structurally characterized by X-ray crystallographic analyses and spectroscopic methods. The Co–P bond lengths in both isomers were shorter than those in the analogous cobalt(III)–bis(tertiary phosphine) complexes with sterically less bulky but more basic phosphine ligands: Co–P(1) = 2.2340(6) and Co–P(2) = 2.2258(7) A for the cis-isomer, and Co–P = 2.276(1) A for the trans isomer. The title complexes also exhibited a unique dynamic behavior: cis to trans isomerization was induced by irradiation with visible light, while thermal trans to cis isomerization took place at elevated temperatures. The absorbance change for the trans to cis isomerization reaction exhibited multi-exponential kinetic traces when no free PHPh2 was present in the solution. Such a complicated kinetic behavior was explained either by the slow dissociation of coordinated PHPh2 or by the abstraction of a P–H proton from coordinated PHPh2 through an acid–base interaction with trace water in the bulk solvent. By addition of an excess amount of PHPh2, the dissociation of coordinated PHPh2 as well as the basicity of impure water was suppressed, and a first-order kinetic trace was observed. Kinetic studies with excess free PHPh2 in acetonitrile revealed that the isomerization reaction takes place via an intramolecular twist mechanism: ΔH* = 120 ± 1 kJ mol−1 and ΔS* = 50 ± 18 J mol−1 K−1. AOM calculations indicate that the twist mechanism involves a spin state change (1A1g to 5A1′) during the activation process. The importance of the π-acidity of PHPh2 together with the cooperative effect of the spectator ligand (dtc−) was suggested to explain the thermodynamic and kinetic behaviors of these complexes.