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Communications to the Editor 0
Reactions of Mo20d2+ with Mercaptans and Implications for Molybdoenzymes
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Sir: 11 S)\ II LMO - x - not L M Q\ o / M o L ' 0 ' There is evidence to suggest that the molybdenum-con\('") )w taining site(s) in molybdoenzymes interacts with the substrate and is also monomeric in nature. However, all such enzymes 0 0 0 II P,Pl which have been reasonably characterized contain two mo/ I /O\ 11 (iiil lviil lybdenum atoms per mole. Similarly, many molybdenum ,s/MoL \s complexes used as enzyme models exist as dimers but are be/ LMO'o/MOL lieved to cleave to produce catalytically active monomers.'-3 W e have investigated the reactions of the Mo20d2+ unit ;I : , s , I x LMo MOL two' in di-p-oxo-bis[oxo-N,N-diethyldithiocarbamatomolybde::( 's' num(V)], [MoOL]202 (ref 4), as probes for elucidating the II pathways by which these bridged systems may be modified or t p o - 0- M o t 2 cleaved (see Figure 1). Three distinct reaction types occur: (a) Figure 1. Reactions of Mo204L2 and interrelationships of products: (i) substitution of bridging and/or terminal oxo groups, with the H2S (1 mol) in CH2C12 at 25'; (ii) excess H2S in boiling CHC13; (iii) structure remaining intact; (b) bridge modification reactions; excess H2S in 1,2-C2H4C12 at 80 OC in bomb; (iv) P4Slo in boiling xylene; and (c) bridge cleavage to produce monomers. The last reac(v) NaL or R2PS2H in 1:l CHCI3/MeOH at 25'; (vi) excess ortho-substituted benzenethiol [o-C6H4(XH)SH; X = S , NH, N(CH3)] in CHzClz tion may remove either bridging oxo groups to produce penat 25'; (vii) o-mercaptophenol (2 mol) in CH2C12 at 25'; (viii) otacoordinate oxomolybdenum(V) monomers or all oxo groups C6H4(XH)SH [X = 0,N(CH3)] ( 1 mol), excess ~ - C ~ H J ( S H ) C Oor~ H to produce hexacoordinate molybdenum(V) monorner~.~ Such excess 1,2-C2H4(SH)OH in CH2C12 at 25 OC; (ix) C5H5N/H20 in bridge cleavage reactions may be important in both the natural CHC13 at 25 OC; (x) excess RSH (R = Me, Et, Ph) in CHzClz at 25 and model catalytic systems. A critical feature of all these new "C. reactions is the requirement for sulfur on the incoming rebenzenethiols (reaction vi) in CHzClz at 25 "C gives a second agent. type of monomeric species MoL(o-C6H&S)2 ( x = s, NH, Reactions of type (a) occur with H2S and P4S10. One p-oxo N(CH3)15). Similar complexes have been postulated to exist group only is replaced by p-sulfido on reaction with H2S (1 in solution (from EPR studies16) but have not previously been mol) at ambient temperature in chloroform solution6 (reaction isolated and characterized. The amino proton was found to be i in Figure l ) , while excess H2S produces mixtures of 1-0sand labile, exchanging rapidly with MeOD in CH2C12.l' Chemical p-& complexes. However, in boiling chloroform, only and EPR studies on these products are reported separate[MoOL]20S is produced even with excess H2S (reaction ii). ly.18 In H2S-saturated, 1,2-dichIoroethanesolution a t 80" in a bomb Various interconversions are possible and those for (reaction iii), complete substitution of p-oxo or di-p-oxo groups [MoYL]2X2 (Y = 0, X2 = 0 2 , S2, OS; and Y = X = S) are occurs. With P ~ S inI ~boiling xylene, both terminal and shown in Figure 1. Successive substitution of oxo by sulfido bridging (when present) oxo groups are replaced giving occurs until [MoSL]2S2 is produced. This last species is inert [MoSL]& (reaction iv).8 Extraction with CH2Cl2 and to 0 2 and H20 and, as yet, the bridging and/or terminal sulcrystallization gives [MoSL]& (32% yield) in an improved fido groups have not been removed from such compounds procedure over that originally r e p ~ r t e d . ~ without causing extensive decomposition. MoOL(o-C6H40S) Two bridge modification reactions (type b) occur, both rereacts with excess o-dimercaptobenzene or o-aminobensulting in the loss of one p-oxo group. Reaction v with QH (Q zenethiol in CH2C12 at 25 OC to give MoL(o-C6H4XS)2 (X = R2NCS2,' R2PS2) in 1:l CHC13-CH30H a t 25" gives = S or N H ) (reaction vi). However, [ M o O L ] ~ ~ ( ~ - C ~ H ~ ~ S ) [MoOLQ]20, while thiols (RSH; R = Me, Et, Ph) in dichlodoes not give MoOL(o-C6H40S) with excess o-mercaptoromethane givelo [MoOL]20(SR)2 (see reaction x in Figure phenol; i.e., cleavage to monomers surprisingly does not occur, 1). A bridging environment for the mercaptide groups is possuggesting that the triply-bridged complex is not an intermetulated based on spectroscopic observations.' Similar diate in the cleavage pathway. H2S (1 mol) does not react with "three-atom-bridged'' products are also formed with excess M o L ( o - C ~ H & ) ~or with [MoOL]20X (X = 1,2-C~H40S, 2-mercaptoethanol and o-mercaptobenzoic acid and with O-C6H&S) under argon in CH2Cl2 within 20 h at 25 O C , but N-methylaminobenzenethiol (1 mol) and o-mercaptophenol the similar reaction with [MoOL]2O(SR)2 produces (1 mol), where each acts as a p2 [either p2-OSor p2-SN(CH3)] [MoOL]20S and R S H (2 mol) (reaction i). Analogous reac-system (reaction viii). N o reaction occurs with phenol, otivity toward hydrolysis is also found, with only [Mophenylenediamine, dimethylglyoxime, o-aminophenol, or with succinic, maleic, and o-hydroxybenzoic acids. Support for this OL]20(SR)2 hydrolyzing (slowly) in wet chloroform a t 25 "C "three-atom-bridged" system comes from a recent x-ray to produce [MoOL]202. The hydrolysis occurs rapidly if a few crystallographic study of a very closely related quinolin-8-olato drops of pyridine are added (reaction ix). The remarkable outcome from these experiments is the ease complex. I with which oxo groups are removed from molybdenum. Oxo Complete cleavage of the di-p-oxo bridge (type c) occurs groups have been removed by atom transfer from cis-dioxowith o-mercaptophenol(2 mol) in CH2Clz a t 25" to give -20% molybdenum(V1) by reaction with tertiary phosphinesI9 and yield14 of pentacoordinate MoOL(o-C6H40S) with v(M-0) a t 930 cm-I, in addition to some [ M o O L ] ~ O ( O - C ~ H ~ ~ Sfrom ) molybdenum(1V) and -(VI) by protonation with strong acids.20-22The reactions reported herein show that acidity per (reaction vii). An excess of some other ortho-substituted ( X ) /
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