Solution and Nujol Matrix Photochemistry of (η⁵-C₅H₅)₂Os₂(CO)₄ and Nujol Matrix Photochemistry of (η⁵-C₅H₄CH₃)₂Ru₂(CO)₄ and (η⁵-C₅H₅)M(CO)₂H, Where M = Ru and Os

Abstract

Lowering the temperature of a Nujol solution of (η 5 -C 5 H 4 CH 3 ) 2 Ru 2 (CO) 4 containing bands of the trans-and cis-bridging and trans-nonbridging isomers from room temperature to ca. 90 K results in complete disappearance of the bands of the trans-terminal isomer. The photochemistry of (η 5 -C 5 H 4 CH 3 ) 2 Ru 2 (CO) 4 in frozen Nujol at 90 K differs subtly from that of (η 5 -C 5 H 5 ) 2 Ru 2 (CO) 4 in inert gas matrixes at 12 K in that the trans-bridging isomer is found to undergo photolysis at high energy (λ irr > 250 nm) to yield both the triply bridged carbonylloss species, (η 5 -C 5 H 4 CH 3 ) 2 Ru 2 (µ-CO) 3 , and the cis-bridged species isomer. There is no evidence for formation of the trans-nonbridged isomer upon photolysis as observed for (η 5 -C 5 H 5 ) 2 Ru 2 (CO) 4 . The solution photochemistry of (η 5 -C 5 H 5 ) 2 Os 2 (CO) 4 is shown to closely parallel that of (η 5 -C 5 H 5 ) 2 Ru 2 (CO) 4 . Photolysis of (η 5 -C 5 H 5 ) 2 Os 2 (CO) 4 in CHCl 3 yields (η 5 -C 5 H 5 )Os(CO) 2 Cl, while photolysis in benzene yields hydride products believed to arise from metal-to-ring coupling of radicals. Photolysis of (η 5 -C 5 H 5 ) 2 Os 2 (CO) 4 , which exists only in a nonbridged form, in frozen Nujol (90 K) results in CO loss and formation of (η 5 -C 5 H 5 ) 2 Os 2 -(µ-CO) 3 as the only photoproduct. Photolysis of (η 5 -C 5 H 5 )M(CO) 2 H, where M ) Ru or Os, in frozen Nujol yields (η 5 -C 5 H 5 )M(CO)H and a second product that appears to be either (η 5 -C 5 H 6 )M(CO) 2 or the (η 5 -C 5 H 5 )M(CO) 2 radical.

Photochemistry of Transition Metal Hydrides

“…Interestingly, the dicarbonyl analogue RuHCl­(CO) 2 (PPh 3 ) 2 did not show any photoinduced ligand-elimination reaction but underwent reversible photoisomerization according to UV and IR spectra . Irradiation of CpRuH­(CO) 2 in frozen nujol yielded [CpRuH­(CO)], again displaying CO photodissociation . Surprisingly, it was observed that triethylsilane reductive elimination competes with CO photorelease in RuH­(SiEt 3 )­(CO) 3 (PPh 3 ), for which the authors postulated an excited state similar to that of Ru–H 2 species due to the parallel to the corresponding oxidative addition reaction.…”

“…If the temperature is lowered to 4 K, however, pure H 2 can be condensed successfully . Several studies of metal hydride complexes have also been reported using hydrocarbon matrices at 77 K in conjunction with IR spectroscopy for compounds that cannot be sublimed but are soluble in hydrocarbons …”

“…Irradiation of Cp*OsH­(CO) 2 in the presence of H 2 delivered the photoproduct Cp*Os­(H) 3 (CO) formed from CO loss . In agreement with these observations, the photolysis of CpOsH­(CO) 2 in frozen nujol yielded the CO-loss fragment and a species that was speculated to be either the [CpOs­(CO) 2 ] radical or a compound where the hydrogen atom had migrated onto the Cp ring . Photolysis of the mesitylene complex (η 6 -C 6 H 3 Me 3 )­OsH­(CO)­(CH 3 ) in an argon matrix resulted in loss of methane identified by its IR bands and formation of [(η 6 -C 6 H 3 Me 3 )­Os­(CO)] (see below for the photochemistry of the dihydride complex) .…”

Comparison of the One-Electron Oxidations of CO-Bridged vs Unbridged Bimetallic Complexes: Electron-Transfer Chemistry of Os2Cp2(CO)4 and Os2Cp*2(μ-CO)2(CO)2 (Cp = η5-C5H5, Cp* = η5-C5Me5)

“…In general, the carbonyl bridged structures are thermodynamically favored for both the Fe and Ru systems, but significant contributions from the unbridged (all-terminal) isomers may be present, depending on the medium as well as any substituents on the cyclopentadienyl rings. Whereas all-terminal isomers contribute less than 1% to the solution makeup of Fe 2 Cp 2 (CO) 4 , , the ruthenium analogue Ru 2 Cp 2 (CO) 4 displays more balanced proportions of the bridged and unbridged structures, with as many as three of the four possible isomers being observed by temperature-dependent infrared (IR) spectroscopy. , In contrast, equilibria between bridged and unbridged isomers have not been observed for the osmium analogues. Thus, the Cp complex 1 retains its all-terminal structures, anti - 1 and gauche - 1 , while undergoing an increase in contribution of the more polar gauche isomer in higher polarity solvents .…”

Primary Photochemical Dynamics of a Triply-Bonded Metal Carbonyl Dimer Probed Via Ultrafast Infrared Spectroscopy

“…4 The photochemical reactivity of homoleptic metal carbonyl dimers of Mn, [5][6][7][8] Re, [9][10][11] and Co, 12 have been characterized in detail, as has that of the cyclopentadienyl carbonyl-complexes of Mo, 13,14 W, 15,16 Fe, [17][18][19][20][21] and Ru. [22][23][24][25] Only a limited number of investigations have been made into the photochemistry of tetracarbonyl group VI dimers, [CpM(CO) 2 ] 2 . The structures of these complexes are distinct from those of the other aforementioned metal carbonyl dimers in that the group VI tetracarbonyl dimers are characterized by higher order M-M bonds of bond order three.…”