Fourier transform 13C nmr study

А FOURIER TRANSFORМ 1 3 С NМR STUDY OF 77-(2,З,5,6-TETRA­ MEТНYL-1,4-BENZOQUINONE) COMPLEXES OF NICКEL, COBALT, RНODIUМ AND IRIDIUМ �цmmary 13С NМR spectra are obtained for а series of quinones, hydroquinones and quinonoid complexes of Ni, Со, Rh and lr iл CDClз, EtOH/D 2 O and D 2 SO4 solutions. Evidence is presented which suggests that neutral 774-duroquinone complexes of the type (С ,оН, 202 )М(С 5 Н 5 ) are protonated in strongly acidic media to produce dications, [(С,оН 1 4O2)М(С5Н5)] 2+ . in which the quinone functions as а 716-hydroduroquinone l.igand. lntroduction Quiлonoid derivatives undergo reaction to produce transition metal com­ plexes of an unusually diverse nature. 2,3,5,6-Tetramethyl-l,4-benzoquinone (duroquinone) has Ьееп found to function as either а two electron 17-ene ligaлd iл [L2 NiЪDq [1] and [L 2 PtЪDq [2], or as а four electron 77-diene ligand in 17-(C5 H 5)M(Dq) complexes of Со, Rh and Ir [3]. Quinones with low reduction potentials [4] such as 1,4-Ьenzoquinone (Е0 = -0.51, -1.14 V) react with Ni(CO)4 to foпn Ni11 salt.s of the hydroqu inone dianion analogous to quin­ hydrone complex.es (5,6]. 2,3,5-Trimethyl-1,4-Ьenzoquinone (Е0 = -0.75, -1.35 V) reacts with Ni(CO)4 in the absence of а cyclic diene to form а para­ magnetic Ni11 complex (ТМQ)2 Ni in wblch the quinone is formally а five electron 71-donor while in the presence of COD it reacts with Ni(CO)4 to form а paramag:netic (COD)Ni(ТМQ) complex in wblch t.he quinone is foпnal1y а 17-arene hydroquinone dianion ligand [5]. 2,3,5,6-Tetramethyl-l,4-benzoquinone (Е° = -0.84, -1.45 V) reacts with Ni(CO)4 in the presence of COD to foпn а tetrahedral. [7], diamagnetic Ni0 complex in which the quinone is foпnally а four electron 71-diene ligand [ 5,8,9]. 1,2-Benzoquinone derivatives have been found to undergo reaction to produce Ьidentate O-bound 1,2-hydroqu inone ,{3 о�с с�о �=-�==t�Z�M Fig. 1. Prop� struc:ture for 2,З,5,6-tetrameLhyl-1,4-benzoqu.шoпe c:omplexes of Ni. Со and Rb from Q ° ° X-ray crystal.l<:>grapblc: stud.ies. (Dq)NHCOD), а 7.5 ; (Dq)Co(CsH :; ), а= 21 . (1 9 ; (Dq)Rb(C; Н;) ° ° rJ 0 . 23 . (J 9"; (Dq), а = = = = = с, = dianion ligaлd (2,10) while 1,4-benzoquiлone reacts with Co(CN)s3 - to foпn O-bound bridged species [11,12). 2,3,5,6-Tetrachloro-1,4-benzoquinone reacts with L 2 Pt derivatives via а vinylic chlorine abstraction to form complexes con­ tainiлg Pt--C а bonds [2] . Crystallograph ic studies of 77-cyclopentadieny l-77-(2,3,5,6-tetrarnethy 11,4-benzoquinone) complexes of cobalt (13] and rhodium [14) have shown а marked deviation from the D::.h symmetry of the free ligaлd [15) iлdicative of 17-diene complexation (see Fig. 1), ,vhile 17-cyclooctadiene-17-(2,3,5,6-tetra­ metbyl-1,4-benzoquinone)nickel [7] exhiblts only а slight deviation from plaлarity. Schrauzer алd Dewhirst ( 3] have suggested that this distortion arises from а repulsive iлteraction of the metal d orbltals ,vith the С=О portion of the filled 2b 1 u and empty 2b 2 g orbltals of the duroquiлone ljgaлd. Struchkov et al. [14,161 have argued that the magлitude of this distortion is not depen­ dent uрол either steric effects or electronic effects of other ligands, but rather uроп the nature of the transition metaJ. Schrauzer алd Thyret [3,17] have suggested tЬat the carbonyl groups iл (Dq)Co(C5 H 5 ) are protonated in strongly acidic medium. \Ve report herein а study of the 13 С NМR spectra of 2,3,5,6-tetrarnethyl-l,4-benzoquinone com­ plexes of Ni, Со, Rh and Ir iл CDCl 3 and D 2 SO.; solution to test the hypothesis that protonation of the quiлone carbonyls iл sulfuric acid medium ,vould de­ crease the reduction potential of the quinone Hgaлd [ 4] алd thereby lead to the conversion of the 77-diene duroquinone complexes to their 77-arene hydroduro­ quiлone dication anaJogs. Experimental 77-Cyclopen tadienyl-17-( 2,3,5,6-tetramethyl-l,4-benzoquiлone) complexes of rhodium and iridium were prepared from the reaction of 2.0 g of the metal tricbloride trihydrate with 4.0 g of duroquiлone iл refluxing ethanol followed Ьу chromatography on silica gel eluted with 80/20 benzene/methanol алd treat­ ment of the resultant metal duroquinone cbloride with cyclopentadienylthallium in tetrahydroг..1I'ЗI1 solution [3,18) .17-Cyclopentadienyl-17-(2,3,5,6-tetramethyl­ l,4-benzoquinone)cobaJt was prepared from the reaction of 17-(CsHs)Co(CO) 2 with excess duroquiлone iл refluxing xylene followed Ьу chromatography on silica gel eluted with benzene/methanol [17]. 77-l,5-f:yclooctadiene-77-(2,3,5,6tetramethyl-l,4-benzoquiлone)nickel was prepared from the reaction of 3.5 m1 of Ni(CO),: with 2.6 g of duroquinone and 20 m1 of COD in refluxing methylene chloride uлder argon. After 6 h the volatile material was distilled into а d.ry-ice­ cooled vessel, the dark red-brown residue was dissolved in 20 ml of methylene cbloride, filtered, t:oncentrated, and the resultant complex precipitated Ьу the addition of 100 ml of light petroleum ether [ 5,8,9]. Complexes were character­ ized Ьу microanalysis and infrared spectroscopy. 13 С NМR spectra were obtained in CDC1 3 , D 2 SO -i and 95% EtOH/5% D2O solutions оп а Varian Associates XL-100-FT spectrometer operating at 25.16 MHz equipped with an internal deuteriurn lock. The 1 3С l\ТМR chemical shifts in CDC1 3 and EtOH/D2 O solutions were measured relative to the internal solvent resonances and are re:;юrted in ppm downfield from TMS using the followiлg conversions: «S(TMS) = Б(СDС1 3 ) - 76.9 8 ppm, and «S(ТМS) = 5(СН 3СН 2 ОН) - 18.3, ppm. Тhе D 2 SO-i spectra were calibrated with an external СНСlз capillary and the chemical shifts are reported in ppm downfield from TMS using the conversion: li(Tl\11S) = 5(CHCl 3)-77.1 8 pprn. All data are the result of at least two indepen­ dent measurements \vith а precision of :::0.06 ppm. Resu.Its С NMR chemical shift data for а series of quinones, hydroquinones and quinonoid complexes are given in ТаЫе 1 and presented graphically in Fig. 2. Berger and Ricker (19] have reported 13С NMR data for 1,4-benzoquinone and 2,3,5,6-tetrarnethyl-l,4-benzoquinone in complete agreement with our results. Attempts to obtain spectra for 2,3,5,6-tetramethyl-l,4-hydroquinone have failed due to the marginal solubility of this derivative, алd we will therefore use 1.4-benzoquinone and 1,4-hydroquinone as model systems for this study. The reduction of 1,4-benzoquinone to 1,4-hydroquiлone leads to а shielding of the C(l,4) and С(2,3,5,6) resonances of 36.3 5 and 19.2s ppm respectively. The 13 ТдВLЕ 1 13 NMR CHEMICдL SHIFTS FOR QUlNONES дND QUINONE COMPLEXES OF Ni, Со, Rb дND lr с Compound 2.3,5.6-Letramethyl1,4-Ь�nzoquшone 1,4-benzoquшone 1,4--Ь ydroquinone (Dq)Ni(COD)b (Dq)Co(C5H 5) (Dq)Rh(C5H5) (Dq)lr(CsHs> Solvent CDCl3 D2SO4 CDCl3 EtOH/D�O CDCl3 CDC/3 D2SO-i CDClз D§O-i CDCl3 EtOH/D2O D§O-i C(l.4) С(2,З,5,6) СНз -187.37° -191.93 -186.9� -150.60 -153.6<1 -157.24 -131.83 -161.65 -133.60(4) -164.81 -161.8g -130.0g -НО.29 -Нб.45 -136.2� -116.97 -112.ls -91.27 -97.95 -93.51(9)с -100.96(6) -82.97 -85.84 -92.50 -12.16 -12.82 -12.09 -14А9 --9.83 -14.77 -9.2::! -14.86 -14.83 -8.69 -85.41 -89.67 -88.24(1) -90.8g{2) -81.83 -83.8-i -83.40 а Chem1cal shifts in ppm downfield from TMS w1Lh а nrecis:lon of ,О.Об ppm. Ь Dq = 2,3,5,6-LetrameLЬyl• 3 1.4-benzoquinone, COD = 1.5-cvclooctad.lene. с I J(10 3 аь 1 с) coupling constants in Hz, ± 1 Hz. 00:з DqCос р-С DqЖ:OO-CD03 Нqu-EtOН С(2,3,5,бl СО,41 1 -200 1 -180 1 1 -160 ·140 1 -120 J -100 1 -80 ppm 13 F1g. 2. с NMR cbem.icaJ shilts ror qu.шones, bydroqu.шones and qu.шono1d complexes ш ppm down­ fieid from TMS. Dq = 2,3,5,6-LEL.-ameLhyl-1,4-ben.zoquinone, Qu = 1,4--henzoqu.шone, Hqu = 1,4--bydroqui­ пoпe. C(l,4) and С(2,3,5,6) carbon environments in hydroquiлone сал Ье thought to Ье the sum of the (C(l) + С(4)} and (С(2,6) + С(З,5)} carbon environments in phenol. Using substituent parameters for phenol [20] we would predict а de­ shie1ding of the C(l,4) and С(2,3,5,6) resonances in bydroquinone of-19.0 and 10.8 ppm relative to benzene, in accord mth experimentaJ values of-21.9 and 11.7 ppm. MacieI and co-workers have studied solvent effects on the С=О resoпance in organic carbonyls [21-23}. They have observed only slight changes in the carbonyl chemical shift in aprotic solvents whereas the carbonyl chemical shift is directly proportional to the acidity of protic solvents. Maciel argues t.hat the deshielding of the carbonyl resonance with increasing solvent acidity arises from an increase in the С=О clipole upon protonation [21]. Chloroform and sulfuric acid desbleJd the carbonyl resonance in acetone, relative to пеаt acetone, Ьу -2.3 and -39.2 ppm respectively, while the carbonyl resonance in hexachloro­ acetone exhiblts onJy а slight susceptibllity to solvent effects. Macie1 suggests that this is а result of the decreased basicity of the carhonyl in hexachloroacetone due to the electron withdrawing character of the CCI 3 substituents. In sulfuric acid solution we note that both the C(l,4) and С(2,3,5,6) resonances in 2,3,5,6tetramethyl-1,4-benzoquinone are deshielded relative to cbloroform so1ution. Тhе magnitude of this effect would suggest а marginal basicity for the carbonyl groups in the free J.igand. It is therefore of interest to note а marked shielding of the C(l,4) resonance in (Dq)lr(CsHs) with increasing solvent acidity in the order CDClз EtOH D2 SO-i, coupled with а concomitant desblelding of the С(2,3,5,6) resonance. If we defiлe such that: б' = о[С(2,З,5,6)] - li[C(l,4)], then for (Dq )lr(C5 H5 ), decreases from 81.84 ppm in CDCI 3 to 76.04 ppm in EtOH and 37.58 ppm in D 1SO-i, while б' is 50.7з ppm for 1,4-benzoquinone and 33.6 3 ppm for 1,4-hy­ droqwnoпe. ТЬеsе data are consistent with the conversion of the neutral 17-diene 2,3,5,6-tetramethyl-l,4-benzoqulnone complex to а 17-arene 2,3,5,6-tetramethyl1,4-hydroquшone dication in acidic medium. Тhе С(2,3,5,6) resonance in (Dq)Rh(C5 H5 ) in CDCl 3 solution exhiblts nuclear spin----spin coupling with the о• о' < < Rh nucleus while the C{l.4) resonance does not, in accord with the crystallo­ graphic data. In D�S04 solution coupling is observed to both the C(l,4) and С(2,3,5,6) resonances, suggestive of direct overlap of rhodium atomic orbltals with the carbonyl carbon atoms. А slight deshielding (-0.7 ppm)* of the СНз resonance in 2,3,5,6-tetramethyl-l,4-benzoquinone is observed in sulfuric acid solution consistent with, if not iпdicative of, the removal of electron density from the duroquinone ring upon protonation, whereas the СН 3 resonances for the (Dq)M(C 5 H 5 ) complexes are shielded Ьу approximately 5 ppm in sulfuric acid solution as expected from the proposed reduction of the duroquinone ring. Attempts to obtain spectra of (COD)Ni(Dq) in D�S04 solution have failed, possi­ Ыy due to the formation of а paramagnetic material. This would Ье consistent with the kлown paramagnetic character of 77-arene hydroquinone derivatives of Ni' 1 [5]. We have observed а shielding of arene resonances upon complexation in 77-(arene)M(C0) 3 complexes of Cr, Мо and W, and in 77-(С 5Н5 ) 2 М complexes of Fe, Ru and 0s [24,25). If we def'me the complexation chemkal shift to Ье the difference between the chemical shift of the free arene and the chemical shift of the complexed arene, then the comple.xation chemica1 shift increases in the order Мо < Cr < W апd Ru < Fe < 0s. Ал analogous order is observed for the C(l,4), С(2,3,5,6) and С 5 Н 5 resonaпces in (Dq)M(C 5 H 5 ) complexes in D2 S04 solution, Rh < Со < lr, suggestive of а 77-arene complex. In CDC1 3 solution this order is observed for tbe С(2,3,5,6) апd С 5 Н 5 resonances only. The order of increasing C(l,4) resonance chemical shifts for dщoquinone complexes in CDCl 3 solution, Ni < Со< Rh < lr, is in agreement with the order of increasing overlap of traлsition metal d orbltals with the 2Ь,u, Ьзg алd 2Ь2 g orhitals of the duroquinone ring [3] suggesting а deshielding of the С( 1,4) resonance with increasing distor­ tion from planarity. Gubin and Кhandkarova [26) have studied the electrochemical reduction of (Dq)M(C 5 H 5 ) complexes of Rh and Ir iп aprotic solvents and they conclude that complexation leads to а shift of the reduction potentials for duroquinone to more negative values. This is in accord with the caJcuJations of Schrauzer and Dewhirst [3] which indicate а marked increase in the energy of the 2Ь2 g orhital upon complexation. Ощ studies differ in that, first, we used а strongly protic medium and it has been observed that the reduction potentiaJs of quinones are substantially less negative in aqueous solutions even at рН > рК 2 , while the re­ duction potentiaJ becomes positive at very low рН [ 4 ]. Second, the process oЬ­ served Ьу Gubin and Кhandkarova involves the reduction of а formally 18 elec­ tron system with an e.xternal source of electrons, rather than the reduction of the quinone via tbe transfer of electron density from the transition metal to the ligand invoked herein. 103 Acknowledgement We gratefully acknowledge the financial support of the School of Chemical Sciences, University of Пlinois, and fruitful discussions with Dr. L.J. Todd, In­ diana University, and Dr. D.W. Slocum, Southern Шinois University. • It has been suuested that in the ahsence of bulk rnagnetic susceptibihty coпections, cheШJcal sh..lft differences of less Lha.n ODe ppm Ье i.gnored. 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