Synthesis and electrochemical studies of osmium(VI) complexes with five- and six-membered nitrogen-containing heterocycles; the solid-state structures of [OsO 2Cl 2L 2] (L=NC 5H 4-4-CN, N 2C 4H 4, N 2C 3H 4), [OsO 2(N 2C 3H 4) 4](NO 3) 2 and [OsO 2(N 2C 3H 4) 2(μ-N 2C 3H 3)] 2(NO 3) 2

Abstract

The neutral mononuclear osmium(VI) dioxo complexes cis-[OsO 2Cl 2L 2] [L=pyz ( 3a), L=cpy ( 3b); pyz=pyrazine, cpy=4-cyanopyridine] ( cis refers to the Cl functionality) are accessible by treatment of OsO 4 ( 1) with pyz ( 2a) or cpy ( 2b) in a 1:2 molar ratio in a methanol/acetone mixture in presence of HCl. Within this reaction HCl acts as chloride source for the transition metal osmium. When 3a is reacted with 2 equiv. of [Pd(dppp)](OTf) 2 ( 4) (OTf=OSO 2CF 3, dppp=diphenylphosphinopropane) homobimetallic [Pd(dppp)(μ-Cl)] 2(OTf) 2 ( 5) along with pyz ( 2a) is produced upon precipitation of OsO 2. A similar reaction behaviour is observed, when 3a is treated with Mo(CO) 5(Thf) (Thf=tetrahydrofuran) ( 6), since mono- and bimetallic complexes of structural type Mo(CO) 5(pyz) ( 7), [Mo(CO) 5] 2(μ-pyz) ( 8) and Mo(CO) 5Cl ( 9) and further chloride-containing molybdenum species are formed. These results show that 3a acts, both as chloride (complexes 5 and 9) and pyz (complexes 7 and 8) transfer reagent, whereby redox processes play a decisive role. The reaction of 1 with Hpz (Hpz=pyrazole) ( 10) leads to different coordination complexes depending on the acid used: While 1 affords with 10 in a 1:2 molar ratio in presence of HCl a trans/ cis mixture of mononuclear neutral [OsO 2Cl 2(Hpz) 2] ( 11), it is found that, when HNO 3 is used instead of HCl, the cationic mono- and homobimetallic osmium(VI) dioxo complexes [OsO 2(Hpz) 4](NO 3) 2 ( 12) and [OsO 2(Hpz) 2(μ-pz)] 2(NO 3) 2 ( 13) (pz=pyrazolate) are produced. This different chemical behaviour will be discussed in detail. The X-ray structures of 3a, 3b and 11– 13 have been determined. Crystals of 3a are monoclinic, space group P2 1/ n, with a=11.3700(3), b=9.2039(3), c=12.1107(4) Å, β=104.2190(10)°, V=1228.54(7) Å 3, Z=4, and final R 1=0.0350 for 3331 reflections with I≥2 σ( I) and 186 variables. Complex 3b is isostructural with 3a [ 3b: monoclinic, space group P2 1/ c, with a=6.74350(10), b=11.495, c=18.276 Å, β=94.2250(10)°, V=1412.88(2) Å 3, Z=4, and final R 1=0.0482 for 4072 reflections with I≥2 σ( I) and 223 variables]. Crystals of 11 are monoclinic, space group C2/ c, with a=14.5510(8), b=5.6901(3), c=14.2792(8) Å, β=117.079(1)°, V=1052.67(10) Å 3, Z=4, and final R 1=0.0263 for 1504 reflections with I≥2 σ( I) and 82 variables. Crystals of 12 are triclinic [ P 1 ̄ , a=9.6569(12), b=10.3194(14), c=10.367(2) Å, α=81.046(11), β=82.577(12), γ=75.405(12)°, V=983.2(3) Å 3, Z=2, R 1=0.0314, 5461 reflections I≥2 σ( I), 344 variables], while 13 is crystallizing in the monoclinic space group C2/ c, with a=16.6967(19), b=8.9933(10), c=20.254(2) Å, β=106.962(2)°, V=2909.0(6) Å 3, Z=4, and final R 1=0.0623 for 3419 reflections with I≥2 σ( I) and 200 variables. Complexes 3a, 3b and 11 contain a monomeric [OsO 2Cl 2L 2] unit (L=pyz, cpy, Hpz) in which the group-8 transition metal atom is octahedrally coordinated. Complexes 3a and 3b are cis-configurated, while 11 is trans-orientated in the solid-state. A coordination polymer is formed by the formation of hydrogen bridges between the hydrogen atom of the Hpz ligands (NH) with the osmium-bound oxygen atoms of individual [OsO 2Cl 2(Hpz) 2] building blocks. In 12 four pyrazole ligands are datively-bound to a OsO 2 entity, whereby the oxygen atoms occupy the axial and the Hpz ligands the equatorial positions of the OsO 2(Hpz) 4 octahedron. Through the formation of NH⋯ONO 2⋯HN bridges between OsO 2(Hpz) 4 units a polymeric structure is set-up. In homobimetallic 13 two OsO 2(Hpz) 2 moieties are spanned by two pz − units. The NO 3 − ions act, as in 12, as bridging units between the Hpz ligands of individual OsO 2(Hpz) 2 entities by forming NH⋯ONO 2⋯HN structures. In addition, a solvent molecule MeOH is bound to an oxygen atom of a OsO 2(Hpz) 2 unit forming a MeOH⋯OOs building block. The synthesized osmium(VI) complexes allow systematic and comparative studies on their electrochemical behaviour, which also will be discussed in detail.