2129
A Comparison of Isomeric Carbene (Ylide) and Amidine Complexes of Cobalt( 111). An Approach to the Study of Carbon-Coordinated Imidazole Daniel J. Doonan, John E. Parks, and Alan L. Balch* Contribution f r o m the Department of Chemistry. lJnicersity of California. DaGis, California 9561 6. Receiced April 18, 1975
Abstract: The Co(II1) carbene complexes [Co(DMGH)2(L)2] [PF,] and [ C O ( C H ~ ) ( D M G H ) ~ ( L(DMGH )] = dimethylglyoximato, L = :C(NHCH3)2) and the isomeric formamidine complexes (L = :N(CH3)CH(NHCH3)) have been prepared and characterized. These systems are viewed as acyclic analogues of C- and N-coordinated imidazole complexes. Proton magnetic resonance (IH N M R ) spectra indicate that the carbene and amidine ligands both contain planar, conjugated N-C-N units similar to those observed in amidinium ions. Specific assignments of the ligand conformations have been made on the basis of IH N M R parameters. The effects of internal steric interactions and solvents on the observed conformer distributions are discussed. [Co(CH,)(DMGH),(C(NHCH3)2)] undergoes irreversible themal isomerization in solution to form the isomeric formamidine complex. Possible mechanisms for this novel linkage isomerization are considered. Protic acids induce rapid solvolysis of the formamidine ligands in both types of complexes. lJnder similar conditions the carbene complexes remain intact but undergo rapid proton exchange a t the carbene nitrogens.
Recent studies of ruthenium complexes of imidazole have revealed the existence of complexes containing imidazole coordinated through a ruthenium carbon bond, 1, as well as the more common coordination through nitrogen, 2.Iq2
H
readily available through the reaction of amines with coordinated isocyanides. Complexes containing this unit offer a convenient analogue to the C-bound imidazole 1. Amidine complexes 4, which are acyclic analogues of 2, may be prepared by either the reaction of an amine with a coordinated nitrile5s6 or, as reported here, by the direct reaction of an amidine with a metal ion. Herein, the synthesis of low spin cobalt(I11) complexes of types 3 and 4 are reported. Such complexes are of potential relevance to the chemistry of coenzyme B12 which contains a benzimidazole function that acts as one of the axial ligands of the central cobalt.' This work, along with previous preparations of low spin iron( 11) carbene complexes,*-I0 indicates that a t least two common metals have the potential for forming complexes with imidazole through C-2. Although the structural similarities between carbene complexes and free amidinium ions have been emphasized in previous studies,s,' few comparisons between amidine ligands and carbene ligands have been made. In addition to the structural relationships between coordinated carbenes and amidines, these moieties are related in another way. When carbene ligands are separated from a metal, they isomerize into the corresponding formamidine. Consequently, it appears that carbene complexes are intermediates in the metal catalyzed synthesis of formamidines via the CYaddition of amines to isocyanides.'2 In addition to the syntheses of these carbene and amidine complexes, an investigation of the conformation of both types of ligands, the behavior of both types of ligands upon protonation, and an example of the isomerization 3 4 is reported.
H
I
Under acidic conditions it has been shown that some complexes of type 2 undergo isomerization to species of type 1. At present this novel type of linkage isomerism appears to be limited to a few complexes of ruthenium. However, the C-bound linkage isomer is related to a number of other ylide or carbene complexes which have been prepared by other r o ~ t e s . Other ~ . ~ complexes containing bonds from a metal to C-2 of various imidazole derivatives have been prepared, but in these cases the positioning of substituents on both of the imidazole nitrogen atoms has effectively blocked these atoms as potential coordination sites3v4 Because of the frequent occurrence of imidazole units a t the metal binding sites of naturally occurring macroniolecules, it has been suggested] that this form of linkage isomerism may have considerable biological significance. An important question which arises in this context concerns the ability of more common metal ions to engage in binding imidazole through C-2. Since the stability of such species with biologically important metals like iron, cobalt, and copper is not established, it is not certain whether failure to observe C-bound imidazole complexes of these metals is due to their instability or to the lack of a facile synthetic route to the C-bound linkage isomer. In order to explore these questions a rational route t o the preparation of similar linkage isomers has been devised. Acyclic carbene complexes 3 are
'NRH 3
-+
Experimental Section Preparation of Compounds. Methyl isocyanideI3 (caution, toxic). N,N'-dimethylf~rmamidine,'~ methylatobis(dimethy1gly-
oximato)cobalt( 111) dimer,'j and bis(di(methy1amino)carbenelgold(l) hexafluorophosphate (14)12were prepared by the reported procedures. [(CH~NC)~CO(DMGH)~][PF~]. A solution containing 928 nig (8.00 mmol) of dimethylglyoxime in 50 ml of ethanol was added to a solution of 952 mg (4.00 mmol) of cobalt chloride hexahydrate in 5 ml of water. Methyl isocyanide (0.6 ml, 10 mmol) was added to the blue-green solution. An air stream was bubbled through the solution for 20 min. After filtration, 648 mg (4.00 mmol) of ammonium hexafluorophosphate dissolved in 10 ml of water was added to the filtrate. The brown crystalline product was collected by filtration. Purification was achieved by recrystallization from nitro-
R 4
Doonan, Parks, Balch
/
Comparison of Carbene and Aniidine Complexes of'Co(II1)
2130 under vacuum. The residue was recrystallized from chloroformmethane-acetone (yield 750 mg, 41%): conductivity, A = 96 cm2 hexane to give 330 mg (90%) of yellow crystals: ir 3390, 3290, mol-' ohm-'; ir 2270 cm-' ( u c ~ N ) ';H N M R in dimethyId6 sulfoxide, T 7.76 (methyl groups of DMGH), 6.62 (CHjNC), -6.95 3170 (w-H), 1565 (0-0,UN...C...N, ~ N - H ) Anal. . Calcd for ( 0 - H e - 0 ) . Anal. Calcd for C ~ ~ H ~ O C O Fc~, N 27.92; ~ ~ ~H,P : C12H25CoN604: C, 38.30; H, 6.70; N , 22.33. Found: C, 38.36; H, 6.81: N,22.44. 3.91;N,16.28.Found:C,28.15;H,3.74;N, 16.40. {(CH~NH)CHN(CH~)ICO(CH~)(DMGH)~. N,N'-Dimethylforma[{(CH~NH)~CI~CO(DMGH)~][PF~]. An ethanol-water solution midine (0.10 ml, 1.4 mmol) was added dropwise to a stirred suscontaining [ ( C H ~ N C ) ~ C O ( D M G H )was ~ J +prepared via the above pension of 0.4 g (0.66 mmol) of [CH3Co(DMGH)2]2 in 15 ml of procedure. Methylamine (1.0 ml of a 40% aqueous solution) was dichloromethane. Hexane (25 ml) was added to the resulting yeladded to the mixture and aeration was continued for an additional low solution to precipitate the product. Recrystallization from di20 min. The mixture was filtered and a solution of 1.0 g (6.1 chloromethane-hexane yielded 0.42 g (77%) of yellow crystals: ir mmol) of ammonium hexafluorophosphate in 10 ml of water was UN...c...N, ~ N - H ) . Anal. 3330 sh, 3280 (UN-H), 1665, 1560 (UN-0, added to the red-orange filtrate. The crude solid product was colCalcd for C12H25CoN604: C, 38.30; H, 6.70; N, 22.33. Found: c, lected and recrystallized from nitromethane to give 1.O g (49%) of 38.52; H, 6.78; N, 22.15. light yellow crystals: conductivity, A = 95 cm2 mol-' ohm-'; ir U N ...c...~). Anal. Calcd for C ~ ~ H ~ O C O - Physical Measurements. These were performed as outlined pre3365 ( u ~ - H ) .1575 (UN-0, viously.'o Infrared spectra were recorded for powdered solids in FbN804P: C, 29.08; H, 5.23; N, 19.38. Found: C , 28.77; H, 4.76; fluorocarbon (4000-1300 cm-I) or mineral oil (1300-600 cm-I) N, 19.37. mulls. [(CH~NH~)~CO(DMGH)~][PF~]. Methylamine (2 ml of 40% Thermal Isomerization Reactions. Solutions of the complexes in aqueous) was added to a solution prepared by adding 928 mg of diN M R tubes were degassed on a vacuum line and the tubes were methylglyoxime dissolved in 50 ml of ethanol to a solution of 952 then sealed under vacuum. A Kontes N M R tube heater served as a mg of cobalt chloride hexahydrate in 5 ml of water. The solution constant ( * O S 0 ) temperature bath. ' H N M R spectra were recordwas aerated for 30 min and filtered. Addition of 648 mg of ammoed periodically at ambient temperatures (ca. 30"). The samples nium hexafluorophosphate to the filtrate produced orange crystals. underwent no perceptible change in compostion during the times Purification was achieved by recrystallization from nitromethane (yield 1.9 g, 96%): conductivity, A = 92 cm2 mol-' ohm-'; ir required to obtain the spectra. 6 ~ - H ) ; 'H 3300, 3220, 3140 ( U N - H ) , 1595, 1570, 1560 sh (UN-0, Results and Discussion N M R in acetone-d6, 7 8.40 (CH3N, triplet, 'JH-H = 6.5 Hz), 7.86 (methyl groups of DMGH), 7.28 (Hl-N, broad, 3 J H 4 not rePreparation and Characterization of Complexes. T w o solved). Anal. Calcd for C ~ O H ~ ~ C O F ~C, N 27.92; ~ O ~ H, P : 3.91: N, pairs of linkage isomers have been prepared. Both involve 16.28. Found: C, 28.15; H, 3.74; N, 16.40. Co(II1) and utilize two dimethylglyoxime monoanions [{(CH~NH)~C}~CO(DMGBF~)~][BF~]. Boron trifluoride etherate (DMGH) a s in-plane donors. T h e first pair of isomers a r e (2.0 ml) was added to a suspension of 200 mg (0.346 mmol) of cations 5 and 6. T h e two axial sites, L and L', are occupied [ { ( C H ~ N H ) ~ C ) ~ C O ( D M C H ) ~in] [20 P Fml ~ ] of ether. The mixture was stirred at room temperature for 72 h. The solid was then collected, washed with ether, and recrystallized from acetonitrile1-propanol to give 210 mg (96%) of product: ir 3415 (UN-H).1615, 1585, 1569, 1550 (UN-O,UN...c...N, ~ N - H )1065, . 1012 (UB-F). Anal. Calcd for C I ~ H ~ ~ B ~ C O Fc, ~ N27.31; & ~ : H, 4.58; N, 18.20. Found: C , 27.53; H, 4.81; N, 18.02. [{(CH~NH)CHN(CH~)]~CO(DMGH)~][PF~]. N,N'-Dimethylformamidine (2.0 ml, 20 mmol) was added to a solution prepared by mixing a solution of 952 mg of cobalt chloride hexahydrate in 5 ml of water with a solution of 928 mg of dimethylglyoxime in 50 ml of ethanol. Air was bubbled through the solution for 45 min. After filtration 3 ml of 65% aqueous hexafluorophosphoric acid was added to the filtrate. The mixture was stored at -10 "C for 1 h and then filtered to collect the yellow-brown crystalline product. Recrystallization from nitromethane-ether gave 350 mg (19%) of pure material: conductivity, A = 100 cm2 mol-' ohm-'; ir 3650 sh, 3570, 3350 (UN-H),1670, 1560 ( U N ...c...~.U N - 0~ ,N - H ) .Anal. Calcd for C ~ & ~ & O F ~ N & ~C,P :29.08; H, 5.23; N , 19.38. Found: C, 28.64: H, 5.38; N.19.27. (CH~NC)(CH~)CO(DMGH)~. This compound was prepared from [CH$o(DMGH)2]2 and methyl isocyanide as described previously:'6 ir 2202 ( u c - N ) , 1535 ( V N - 0 ) ;' H N M R in dimethyl-& sulfoxide T 9.27 (CH3-Co), 7.92 (methyl groups of DMGH), 6.61 (CNCH3, triplet, 2 J ~= 2.5 - ~Hz), -8.40(0-H-O). (CH~NH~)CO(CH~)(DMGH)~. Anhydrous methylamine was bubbled through a suspension of 0.3 g (OS mmol) of [CH3Co(DMGH)2]2 in I O ml of dichloromethane. The solids dissolved within 1 min to give a clear yellow solution. The product was precipitated by addition of ether (15 ml). Following recrystallization from dichloromethane-ether 310 mg (92%) of dark yellow crystals was obtained: ir 3280 sh, 3240, 3160 (VN-H), 1560 ( U N - 0 , ~ N - H )IH ; N M R in dimethyl-d6 sulfoxide T 9.76 (CH3-Co), 8.24 (CH3N, triplet, 3 J H _ H = 6.5 Hz), 7.91 (methyl groups of DMGH), 7.20 (H2N, broad quartet, 3 5 H - H = 6.5 Hz), -8.45 ( 0 - H - 0 ) . Anal. Calcd for CloH22CoN~04:C, 35.83; H,6.61; N , 20.89. Found: C, 36.31; H, 6.50; N, 21.12. I(CH~NH)~CICO(CH~XDMGH)~. Anhydrous methylamine was passed through a solution of 345 mg of [(CH3NC)(CH3)Co(DMGH)2] in I O ml of CH2C12. The solution color changed from light yellow to orange after 1 min. Addition of methylamine was discontinued and three drops of methyl isocyanide were added. The solution was allowed to stand, protected from light, for 48 h. The light yellow solution was evaporated to dryness
Journal of the American Chemical Society
/
98:8
.+
H3C' CH, 5, X = H; L = L' = C(NHCH,),; 2 = 1 6, X = H L = L' = N(CHJCH(NHCH,); i = 1 7, X = BF,; L = L' = C(NHCHJ,; z = 1 8, X = H L =CHJ; L' = C(NHCH,),; z = 0 9, X = H L = CHJ; L' = N(CH,)CH(Ic'HCH,); z = 0
by di(methy1amino)carbene ligands in 5 a n d isomeric N,N'-dimethylformamidineligands in 6. T h e c a r b e n e complex 5 was prepared in a one-step synthesis which involved the aerial oxidation of Co(I1) in t h e presence of dimethylglyoxime, methyl isocyanide, a n d methylamine. In t h e a b sence of methylamine this procedure gives t h e bisisocyanide complex [ ( C H 3 N C ) 2 C o ( D M G H ) 2 1 f . In t h e preparation of 5 t h e intermediate bisisocyanide complex has undergone nucleophilic addition of methylamine t o each isocyanide donor carbon. Complex 6 was also prepared by oxidation of Co(I1) in t h e presence of dimethylglyoxime a n d , in this case, N,N'-dimethylformamidine.This procedure yielded only ca. 20% of t h e desired product. Since a sufficient q u a n tity of 6 was available for characterization, no effort was m a d e t o improve t h e yield. T h e complexes 5, 6, [ ( C H ~ N C ) ~ C O ( D M G H ) ~ ] + ,a n d [(CH~NH~)~CO( D M G H ) 2 ] + were isolated a s crystalline, analytically pure, hexafluorophosphate salts. Conductivity d a t a from nitrom e t h a n e solutions indicate t h a t each salt is a 1 : 1 electrolyte. T h e infrared spectra of these cations a r e consistent with their formulations a n d clearly indicate t h a t t h e isomers 5 a n d 6 a r e distinguishable. Both isomers show strong N-H stretching vibrations in t h e 3500-3 100 cm-' region a n d a b sorbtions d u e to ~ N - Ha n d v y ...c...N in t h e 1600- 1500 cm-I region. T h e ' H N M R spectra of t h e complexes a r e also in
/ April 14, 1976
2131 35
iAl
H
6 98
7-5,
N-CHS
N-CH3
M-C' \N-H
735
co-c
co-c,
/
CHs
\
E2
N(CH3) H 3 5
