Electronic Structure of a Binuclear Nickel Complex of Relevance to [NiFe] Hydrogenase

Transcription

1 Inog. Chm. 8, 4, Elctonic Stuctu of a Binucla ickl Complx of Rlvanc to [if] Hydognas Mauic van Gastl,*, Jnnif L. Shaw,, Alxand J. Blak, Maco Flos, Matin Schöd,*, Jonathan McMast, and Wolfgang Lubitz*, Max-Planck-Institut fü Bioanoganisch Chmi, Stiftstass 34-36, D-454 Mülhim an d Ruh, Gmany, and School of Chmisty, Th UniVsity of ottingham, ottingham, UK G RD, U.K. Rcivd July 3, 8 Th binucla complx [i (L)(MC) ] 3+ (L - ) compatmntal macocycl incopoating imin and thiolat S donos) has a i III cnt bidgd via two thiolat S-donos to a diamagntic i II cnt. Th gound-stat has dominant 3d z chaact simila to that obsvd fo [if] hydognass in which i III is bidgd via two thiolat donos to a diamagntic cnt (F II ). Th systm has bn studid by X-ay cystallogaphy and puls EPR, ESEEM, and EDOR spctoscopy in od to dtmin th xtnt of spin-dlocalization onto th macocycl L -. Th hypfin coupling constants of six nitogn atoms hav bn idntifid and dividd into th sts of two quivalnt nitogns. Th most stongly coupld nitogn atoms (a iso 53 MHz) stm fom axially bound solvnt actonitil molculs. Th two macocycl nitogns on th i III sid hav a coupling of a iso MHz, and thos on th i II sid hav a coupling of a iso - MHz. Dnsity functional thoy (DFT) calculations confim this assignmnt, whil compaison of th calculatd and xpimntal 4 hypfin coupling constants yilds a complt pictu of th lcton-spin dnsity distibution. In total, 9% spin dnsity is found at th i III of which % is in th 3d z obital and 6% in th 3d obital. Th i II contains -3.5% spin dnsity, and.5% spin dnsity is found at th axial MC ligands. In analogy to hydognass, it bcoms appant that binding of a substat to i at th axial positions causs a distibution of th lcton chag and spin dnsity, and this distibution polaizs th chmical bonds of th axial ligand. Fo [if] hydognass this implis that th H bond bcoms polaizd upon binding of th substat, which may facilitat its htolytic splitting. Intoduction Hydognas nzyms a highly fficint hydognpocssing catalysts that ly on conomically attactiv bas mtals. Th [if] hydognass mdiat hydogn poduction and consumption (q ), and th activ sit of ths nzyms (Figu ) compiss a htobinucla cnt incopoating i and F ions. -6 Th F cnt is coodinatd to th diatomic ligands which hav bn idntifid by FTIR * To whom cospondnc should b addssd. vgastl@ mpi-mulhim.mpg.d (M.v.G.); m.schod@nottingham.ac.uk (M.S.); lubitz@mpi-mulhim.mpg.d (W.L.). Tl: +49 () (M.v.G.); +44 () (M.S.); +49 () (W.L.). Fax: +49 () (M.v.G.); +44 () (M.S.); +49 () (W.L.). Max-Planck-Institut fü Bioanoganisch Chmi. Th Univsity of ottingham. () Fontcilla-Camps, J. C.; Volbda, A.; Cavazza, C.; icolt, Y. Chm. RV.,, studis as two C - ligands and on CO. - Th i and F cnts a bidgd by S atoms fom cystin sidus and, () Volbda, A.; Chaon, M.-H.; Hatchikian, E. C.; Fy, M.; Fontcilla- Camps, J. C. atu 995, 33, (3) Higuchi, Y.; Yasuoka,.; Kakudo, M.; Katsub, Y.; Yagi, T.; Inokuchi, H. J. Biol. Chm. 98, 6, (4) Fontcilla-Camps, J. C. J. Biol. Inog. Chm. 996,, (5) Volbda, A.; Fontcilla-Camps, J. C. Cood. Chm. RV. 5, 49, (6) van Gastl, M.; Lubitz, W. In High Rsolution EPR: Applications to mtallonzyms and mtals in mdicin; Hanson, G. R., Blin, R. J., Eds.; Kluw Acadmic: w Yok, 8; pp () Happ, R. P.; Rosboom, W.; Piik, A. J.; Albacht, S. P. J.; Bagly, K. A. atu 99, 385, 6 6. (8) Blijlvns, B.; van Bokhuizn, F.; D Lacy, A. L.; Rosboom, W.; Fnandz, V. M.; Albacht, S. P. J. J. Biol. Inog. Chm. 4, 9, (9) D Lacy, A. L.; Hatchikian, E. C.; Volbda, A.; Fy, M.; Fontcilla- Camps, J. C.; Fnandz, V. M. J. Am. Chm. Soc. 99, 9, () Fichtn, C.; Lauich, C.; Both, E.; Lubitz, W. Biochmisty 6, 45, Inoganic Chmisty, Vol. 4, o. 4, 8./ic8383x CCC: $4.5 8 Amican Chmical Socity Publishd on Wb /8/8

2 Elctonic Stuctu of a Binucla ickl Complx Figu. X-ay stuctu of a [if] hydognas (lft). Th activ sit of [if] hydognass (ight). Th catalytic activity is blivd to occu at th opn coodination sit of i, indicatd with an aow. in th oxidizd inactiv fom, by an additional spcis (X), which contains ogn.,-5 Th i cnt is coodinatd to two futh S atoms fom cystin sidus (Figu ). Although th a subtl diffncs in th composition of th ligands in th mtal cnts of th catalytically activ [if] hydognass fom diffnt bactia (.g., in som spcis a slnocystin is psnt instad of cystin), th basic stuctu is consvd. 6 H h H () Duing th catalytic cycl th nzym passs though sval dox stats. Thitn stats hav now bn idntifid, 8 svn of which appa to b physiologically lvant with th involvd dictly in catalytic activity. It is undstood that in th catalytic cycl of [if] hydognass th F cnt mains in th fomal F II stat whas th i cnt shuttls btwn th diamagntic + and paamagntic 3+ oxidation stats. -9 Elcton paamagntic sonanc (EPR) studis, thfo, play a ky ol in undstanding th dox chmisty of th activ sit in [if] () Capo, M.; Tiny, D. L.; Bondino, C. D.; Yang, T. C.; Pamplona, A.; Tls, J.; Moua, I.; Moua, J. J. G.; Hoffman, B. M. J. Am. Chm. Soc., 4, () Volbda, A.; Matin, L.; Cavazza, C.; Matho, M.; Fab, B. W.; Rosboom, W.; Albacht, S. P. J.; Gacin, E.; Rousst, M.; Fontcilla- Camps, J. C. J. Biol. Inog. Chm. 5,, (3) Ogata, H.; Hiota, S.; akahaa, A.; Komoi, H.; Shibata,.; Kato, T.; Kano, K.; Higuchi, Y. Stuctu 5, 3, (4) van Gastl, M.; Fichtn, C.; s, F.; Lubitz, W. Biochm. Soc. Tans. 5, 33,. (5) van Gastl, M.; Stin, M.; Bcht, M.; Schöd, O.; Lndzian, F.; Bittl, R.; Ogata, H.; Higuchi, Y.; Lubitz, W. J. Biol. Inog. Chm. 6,, 4 5. (6) Halcow, M. A.; Chistou, G. Chm. RV. 994, 94, () Huytt, J. E.; Capo, M.; Pamplona, A.; Fanco, R.; Moua, I.; Moua, J. J. G.; Hoffman, B. M. J. Am. Chm. Soc. 99, 9, (8) Evans, D. J.; Picktt, C. J. Chm. Soc. RV. 3, 3, (9) Wang, Q.; Baclay, J. E.; Blak, A. J.; Davis, E. S.; Evans, D. J.; Ma, A. C.; McInns, E. J. L.; McMast, J.; Wilson, C.; Schöd, M. Chm. Eu. J. 4,, () Lubitz, W.; Rijs, E.; van Gastl, M. Chm. RV.,, hydognass. Quantum chmical calculations hav shown that th dox chmisty of [if] hydognas is not only limitd to th i cnt; ath, in th EPR activ stats, a significant amount of spin dnsity is found on th S-dono of on of th bidging cystin ligands. Th noninnocnc of th cystin ligands is intiguing, and it has bn suggstd that a possibl ol of th cystin ligands is to act as bass to bind potons duing th catalytic pocss. 4, A ang of low molcula wight inoganic complxs hav bn synthsizd with th aim of undstanding th lctonic stuctu and function of th activ sit of [if] hydognass. 3,4 Excllnt modl systms hav bn potd including mononucla complxs with xclusivly sulfu ligation, binucla complxs with ith idntical o diffnt mtals, and complxs with a vaity of coligands. 5-3 A fw functional bi- and htonucla modl complxs hav bn potd fo hydogn oxidation and poton duction () Stin, M.; Lubitz, W. Phys. Chm. Chm. Phys., 3, () Buschi, M.; Zamplla, G.; Fantucci, P.; D Gioia, L. Cood. Chm. RV. 5, 49, (3) Ma, A. C.; Spnc, D. J. E.; Schöd, M. Cood. Chm. RV., 9, (4) Bouwman, E.; Rdijk, J. Cood. Chm. RV. 5, 49, (5) Li, Z. L.; Ohki, Y.; Tatsumi, K. J. Am. Chm. Soc. 5,, (6) Zhu, W. F.; Ma, A. C.; Wang, Q.; s, F.; Spnc, D. J. E.; Blak, A. J.; Cook, P. A.; Wilson, C.; Schöd, M. Poc. atl. Acad. Sci. U.S.A. 5,, () Stnson, P. A.; Main-Bca, A.; Wilson, C.; Blak, A. J.; McMast, J.; Schöd, M. Chm. Commun. 6, (8) Gn, K..; Jffy, S. P.; Ribnspis, J. H.; Dansboug, M. Y. J. Am. Chm. Soc. 6, 8, (9) Jffy, S. P.; Singlton, M. L.; Ribnspis, J. H.; Dansboug, M. Y. Inog. Chm., 46, (3) Rampsad, M. V.; Jffy, S. P.; Goldn, M. L.; L, J.; Ribnspis, J. H.; Dansboug, D. J.; Dansboug, M. Y. J. Am. Chm. Soc. 5,, (3) Matsumoto, T.; akaya, Y.; Itakua,.; Tatsumi, K. J. Am. Chm. Soc. 8, 3, (3) Ogo, S.; Kab, R.; Uhaa, K.; Ku, B.; ishimua, T.; Mnon, S. C.; Haada, R.; Fukuzumi, S.; Higuchi, Y.; Ohhaa, T.; Tamada, T.; Kuoki, R. Scinc, 36, Inoganic Chmisty, Vol. 4, o. 4, 8 689

3 van Gastl t al. Figu. Stuctus of [i (L)] + and [i (L)(MC) ] 3+. Th g z and molcula z axs a paalll to th (MC)-i dictions of th local octahdon at th i III cnt as indicatd by an aow. In th latt complx, th fomal valnc stats of th mtals a i II and i III. Fo simplicity, both sonanc stuctus, which hav a foldd confomation, a dawn plana. Th foldd stuctus a givn in Figus 3 and 8. To futh invstigat th dg of dlocalization of spin dnsity ov i and its ligands in a i III systm in a low molcula wight modl, w dcidd to study th binucla complx [i (L)] + (Figu ) 3,38 by pulsd EPR, lcton nucla doubl sonanc (EDOR), lcton spin cho nvlop modulation (ESEEM), and dnsity functional thoy (DFT) calculations. Th oxidizd complx cation [i (L)(MC) ] 3+, fomd by on-lcton oxidation of [i (L)] + in MC, is a valnc-localizd i II /i III systm chaactizd by optical absoption bands at 5 and 865 nm and g valus at.35 (g z ),.8 (g y ), and. (g x ). 39 Th macocycl [L] - is symmtic and nonplana and contains two mtal binding sits. Ths ach compis two tminal and two bidging thiolat goups. Though th complx [i (L)(MC) ] 3+ dos not accuatly mimic th gomtic stuctu of [if] hydognass, its lctonic stuctu, as xpssd by th g valus, is simila to th i-a, i-b, i-c, and i-l paamagntic intmdiat stats of hydognas. Initial studis by multifquncy EPR spctoscopy hav shown that [i (L)(MC) ] 3+ displays a wll-solvd pntt stuctu in th g z gion with a ::3:: intnsity distibution, which was attibutd to th 4 hypfin intaction of solvnt MC ligands in th axial positions. 39 Most pobably, th binding of axial solvnt molculs contibuts substantially to th localizd-valnc i II /i III chaact of th systm. Owing to th foldd natu of th macocycl on coodination, it is not stically possibl fo both mtals to possss two axially coodinatd solvnt molculs coodinatd in a symmtic fashion. Th 3d obitals of th i cnt with two axial ligands a mo dstabilizd than thos of th oth mtal, sulting in th localizd-valnc natu of th systm. Th dox potntials obsvd fo hydognas a also compatibl with a i II / (33) Malli, C.; Rauchfuss, T. B. Angw. Chm., Int. Ed., 46, (34) Justic, A. K.; Linck, R. C.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chm. Soc. 4, 6, (35) Sllmann, D.; Laudbach, F.; Gipl, F.; Hinmann, F. W.; Moll, M. Angw. Chm., Int. Ed. 4, 43, (36) Pa, A.; Davis, E. S.; Hyd, J. R.; Wang, Q.; McMast, J.; Schöd, M. Chm. Commun. 6, 3 5. (3) Atkins, A. J.; Black, D.; Blak, A. J.; Main-Bca, A.; Pasons, S.; Ruiz-Ramiz, L.; Schöd, M. Chm. Commun. 996, (38) Atkins, A. J.; Blak, A. J.; Schöd, M. J. Chm. Soc. Chm. Comm. 993, (39) Banscomb,. D. J.; Atkins, A. J.; Main-Bca, A.; McInns, E. J. L.; Mabbs, F. E.; McMast, J.; Schöd, M. Chm. Commun. 3, i III dox coupl in th psnc of lcton-ich, polaizabl anionic sulfu ligands that stabiliz th i III fom. 4,4-4 By studying [i (L)(MC) ] 3+ sampls nichd with 6 i (I ) 3/) at vaious micowav fquncis, th 6 i hypfin intactions ( A x ) 5 MHz, A y ) 48 MHz, and A z ) 5 MHz) 39 hav bn dtmind. Ths hypfin couplings allow an stimation of th amount of i chaact in th singly occupid molcula obital (SOMO). 39 Howv, th maining pat of th SOMO has yt to b idntifid. W pot hin futh invstigations of th spin dnsity distibution in [i (L)(MC) ] 3+ by pulsd EPR spctoscopy and latd tchniqus, in od to povid a complt dsciption of th SOMO and to povid futh insight into and to lan mo about th mchanism of htolytic dihydogn splitting in [if] hydognass. Expimntal Sction Pchloat salts can b xplosiv if dy. Ca was takn in th synthsis of th pchloat salts dscibd blow not to Vapoat to dynss o hat any solid fomd. Ppaation of Complxs. [i (L)](O 3 ). [i (L)](O 3 ) was synthsizd via an adaptation of th litatu mthod. 3,38,43,6- Difomyl-4-mthyl-,-dimthylcabamylthiophnol (5 mg,.3 mmol) was dissolvd in iso-popanol (5 cm 3 ) and hatd to flux und. aoh (8 mg,. mmol) was addd to th fluxing solution which undwnt a colo chang fom yllow to dak d. Th solution was hatd to flux fo 5 h bfo i(o 3 ) 6H O (589 mg,.3 mmol) and H (CH ) 3 H (. cm 3,.3 mmol) w addd. Th solution was hatd to flux fo a futh 4 h and filtd whil still hot. Th d solid was collctd and cystallizd fom MC and Et O to giv a dak d micocystallin powd. Yild: mg (38%). H MR (CD 3 O ) δ/ppm: 8. (s, 4H),.56 (s, 4H), 4.4 (t, 4H), 3.83 (d, 4H),.38 (s, 6H),. (d, H),.8 (q, H). ES MS m/z: 5 ([M] + ), 549 ([M - H] + ). [i (L)(BF 4 ) ]. Cud [i (L)](O 3 ) ( mg, 34 mmol) obtaind in th pvious action was puifid by Soxhlt xtaction into MC (35 cm 3 ). At th sam tim, anion mtathsis was caid out by hating th solution to flux with an xcss of Bu 4 BF 4 (66 mg, 85 mmol). Th poduct fomd as d cystals (4) Blak, A. J.; Gould, R. O.; Halcow, M. A.; Hold, A. J.; Hyd, T. I.; Schöd, M. J. Chm. Soc., Dalton Tans. 99, (4) Fox, S.; Wang, Y.; Silv, A.; Milla, M. J. Am. Chm. Soc. 99,, (4) Kug, H. J.; Png, G.; Holm, R. H. Inog. Chm. 99, 3, (43) Book, S.; Couch, P. D.; Roxbugh, F. M. J. Chm. Soc., Dalton Tans. 996, Inoganic Chmisty, Vol. 4, o. 4, 8

4 Elctonic Stuctu of a Binucla ickl Complx Figu 3. (a) Viw of [(i (L)ClO 4) (µ -Cl) ] x+ (x ) -) as displacmnt llipsoid plots at 5% pobability. H atoms a omittd fo claity. (b) Coodination gomty of th i cnts. Labls of th atoms a givn in th figu and dtails of th stuctu a givn in th Suppoting Infomation. upon th addition of Et O to th coold action mixtu. Th cystal stuctu of [i (L)(PF 6 ) ] has bn potd in f 38. Yild: 89 mg (83%),. H MR (CD 3 O ) δ/ppm: 8. (s, 4H),.5 (s, 4H), 4.44 (t, 4H), 4.9 (d, 4H),.38 (s, 6H),. (d, H),.8 (q, H). IR (KB) νj max /cm - : 9m (C-H sttch), 894m (CsH sttch), 6w (Cd sttch), 556w, 33s, 5s (BsF sttch), 88m, 85m. [i (L)(MC) ] 3+. A potion of [i (L)](BF 4 ) was dissolvd in analytical gad MC (Mck) and oxidizd by addition of HClO 4 (%). Th final concntation was ca. mm. [(i (L)ClO 4 ) (µ -Cl) ] x+ (x ) -). Singl cystals of [(i (L) ClO 4 ) (µ -Cl) ] x+ (x ) -) w obtaind by adding HClO 4 (%) to dak d cystals of [i (L)](BF 4 ). Ca was takn not to dissolv th cystals. Vy dak gn cystals, which w unstabl xcpt in acidic mdia, w obtaind aft on wk. Ths w th only cystallizabl oxidation poducts obtaind fom a sis of oxidation xpimnts. Unfotunatly, und th xpimntal conditions, additional masumnts to dtmin th chag balanc within th unit cll w not fasibl, and consquntly, th valu of x could not b stablishd with ctainty. Cystal Stuctu Dtmination. Diffaction data w masud at 5 K as ω scans using Mo KR adiation (.3 Å). Th stuctu was solvd by dict mthods using SHELXS, and full-matix last-squas finmnts on F w caid out using SHELXL. Th hydogn atoms of th wat molculs O9-O36 w not locatd, but thy a includd in th unit cll contnts and in all paamts divd fom thm. Comput pogams usd a as follows: Buk SMART, Buk SAIT, and Buk SHELXTL (Buk AXS Inc., Madison, WI, ); SHELXS9; 44 SHELXL9; 45 ncif; 46 and PLATO. 4 Th data fo th cystal and stuctual finmnt a givn in Tabl S, Suppoting Infomation. EPR, EDOR, and ESEEM. Continuous wav EPR spcta w codd on a 9 GHz Buk Elxsys 5E spctomt with a ctangula 4ST cavity and an Oxfod ESR9 flow cyostat. Th tmpatu was K, micowav fquncy GHz, micowav pow mw, and modulation amplitud 5 G. Q-band pulsd EPR and Davis EDOR xpimnts w pfomd at T ) K on a Buk Elxsys FT-EPR systm at 34 GHz and a hom-built EDOR cavity. In th Davis EDOR xpimnts, th fist π puls had a lngth of ns cosponding to an xcitation bandwidth of.8 mt, and th lngth of th RF puls was 8 µs. A Hahn cho dtction squnc of ns was usd fo th EDOR xpimnt. Th cw EPR spcta w simulatd by a hom wittn pogam mad availabl by Pof. F. s. Th pocdu usd fo simulation of th EDOR spcta was adaptd fom an ESEEM simulation pocdu dscibd pviously. 48 Th bands in th fquncy histogam a dssd with a Gaussian function with a width of. MHz. X-band 3 puls ESEEM and Davis EDOR spcta w masud on a Buk Elxsys E58 spctomt in a dilctic ing sonato (ESP 38-5 DL Q-H) using an Oxfod CF935 flow cyostat. Th 3 puls ESEEM xpimnts w codd at T ) K and ν mw ) 9.8 GHz; th π/ micowav pulss of th π/-τ-π/-t-π/-τ-cho squnc had a lngth of 6 ns, and th tim τ was 44 ns. Aft subtaction of th T cho dcay, th modulation pattn was multiplid with a Hamming window function, zo-filld to 4 points and Foui tansfomd to th fquncy domain. Th dpictd ESEEM spcta psntd a magnitud spcta. Davis EDOR spcta w codd with a π-π RF -π/-τ-π-cho squnc. Th lngth of th RF puls was µs, and th tim τ was 4 ns. In this squnc, th lngth of th π puls was ns. In EDOR as wll as ESEEM spctoscopy, two signals a xpctd fo a spin / nuclus (.g., H). If a ff is th ffctiv hypfin coupling constant and ν Z is th Zman fquncy, thn ths signals occu at 49 ν Z ( a ff fo wakly coupling nucli with ν Z > a ff / o (44) Shldick, G. M. Acta Cystallog., Sct. A 99, 46, (45) Shldick, G. M. SHELXL-9; Univstiy of Göttingn: Göttingn, Gmany, 99. (46) Alln, F. H.; Johnson, O.; Shilds, G. P.; Smith, B. R.; Towl, M. J. Appl. Cystallog. 4, 3, (4) Spk, A. L. J. Appl. Cystallog. 3, 36, 3. (48) van Gastl, M.; Comans, J. W. A.; Jukn, L. J. C.; Cants, G. W.; Gonn, E. J. J. J. Phys. Chm. A 998,, (49) Schwig, A; Jschk, G. Pincipls of Puls Elcton Paamagntic Rsonanc; Oxfod Univsity Pss: Oxfod,. () Inoganic Chmisty, Vol. 4, o. 4, 8 69

5 a ff ( ν Z (3) fo stongly coupld nucli with ν Z < a ff /. In cas of nucli with spin I > / (.g., 4, I ) ) additional splitting of ach of ths signals into two may occu owing to th psnc of nucla quadupol intaction. In th cas of a small quadupol intaction, P ff < ν Z, a ff /, th quations hav to b modifid by an additional tm 49 (3P ff (4) Fo nitogn atoms fo which ν Z a ff /, fo xampl, fo th wakly coupld nitogn atoms (vid infa), th nucla hypfin intaction and Zman intaction cancl ach oth in on M S manifold. 5 Thn in th ESEEM spcta, th lativly shap signals occu at ν P z (3 ( η), ν ) P z η (5) wh th asymmty paamt η is dfind as 49 η ) P x - P y P z (6) Of th oth M S manifold, only th doubl quantum ( M I ) ) tansition is obsvd at a fquncy 49 ν dq ) [( a ff + ν n) + ( P ff (3 + η )) ] / Dnsity Functional Thoy (DFT) Studis. DFT calculations w pfomd using th ORCA pogam. 5 Th complx [i (L)(MC) ] 3+ (Figu ) incopoating two axially coodinatd MC molculs bound to th i III cnt only was usd as th modl gomty. Th coodinats a givn in Suppoting Infomation. Th calculations usd th B3LYP functional and a spinunstictd fomalism. Fo light atoms th iglo-iii basis st of Kutzlnigg t al. 5 was usd, whil fo i th cp basis st 53 was usd augmntd with two polaization functions 54 and on diffus function. Hypfin coupling constants w calculatd in a coupldptubd SCF fomalism. Convgnc citia w -6 Eh fo th chang in ngy and -5 and -6 fo th avag and maximum chang in th lcton dnsity, spctivly. In spinunstictd calculations, ngativ spin dnsity may ais as a sult of spin polaization. Spin polaization occus bcaus th xchang intaction only occus btwn lctons of qual spin. This mans that th two lctons of ach lcton pai hav a diffnt intaction with th unpaid lcton. Th slight inquivalnc of th two lctons within th pai causs thi wav functions to b slightly diffnt, thus intoducing small additional positiv and ngativ spin dnsity. In od to invstigat th ffct of polaization of th H bond upon binding to i in th activ sit of [if] hydognas, th additional calculations hav bn pfomd. Th fist involvs a calculation of [i (L)(MC)(H )] 3+, wh on of th MC ligands has bn placd by H. Sinc th i atom in hydognas (5) Flanagan, H. L.; Singl, D. J. J. Chm. Phys. 98, 8, (5) s, F. ORCA, An ab initio, Dnsity Functional and Smimpiical pogam packag, Vsion.5; Univsity of Bonn, 6. (5) Kutzlnigg, W.; Flisch, U.; Schindl, M. MR Basic Pincipls and Pogss; Sping-Vlag: Hidlbg, 99. (53) s, F. Inog. Chim. Acta, 33C, 8 9. (54) Wachts, A. J. J. Chm. Phys. 9, 5, () van Gastl t al. is fiv coodinat with fou S and on O coodination and has on opn coodination sit fo H, th scond modl consistd of [i 3+ (SH - ) 4 (OH - )(H )]. Th thid modl taks into account th complt activ sit of hydognas. A pictoial psntation of all th systms is includd in th Suppoting Infomation. Rsults and Discussion CystalStuctu.Thcystalstuctuof[(i (L)ClO 4 ) (µ - Cl) ] (ClO 4 ) 5H O confims that a ttanucla spcis foms fom two [i (L)] units bidgd by two chloid ions, with th i cnts in ach dg-shaing bioctahdal [i (L)] unit bidgd by a pchloat anion. Th asymmtic unit compiss half of th ttanucla complx cation with th oth half gnatd by invsion. Each i ion is six coodinat and bound to two nitogn atoms and two sulfu atoms of th macocycl, a bidging chloid ion, and an ogn atom fom a bidging pchloat anion (Figu 3). Th i- bond lngths a.949(5),.946(5),.96(4), and.96(4) Å, and th th i-s bond lngths a.3(5),.38(5),.345(5), and.45(5) Å. Ths bond lngths a significantly long than thos obsvd fo th squa plana i II i II spcis [i(l)](pf 6 ) DMF wh th i- bond lngths a.9(5) and.96(5) Å and th i-s bond lngths a.8(6) and.(6) Å. 38 Th i-cl bond lngths a.46(5) and.399(5) Å and th i-o bond lngths a.85(4) and.(4) Å. It was not possibl to assign fomal oxidation stats of i basd on th stoichiomty of th complx as th a six additional pchloat countanions p ttanucla fagmnt idntifid in th cystal stuctu. Du to th difficulty of accuatly locating H atoms in a cystal stuctu, th xact chag of th complx is not known. An assignmnt to i III is favod basd on th bond lngths. A sach of th litatu sultd in th idntification of just th sixcoodinat i III complxs with at last on S dono. 4,55,56 In ths complxs th avag i III -S bond lngths a.3,.3, and.8 Å, spctivly. Th last valu in paticula cosponds wll to th avag i-s bond lngth of.38(4) Å in [(i (L)ClO 4 ) (µ -Cl) ] (ClO 4 ) 5H O. EPR Spctoscopy. Th continuous wav (cw) EPR spctum of a fozn solution of [i (L)(MC) ] 3+ fomd by th oxidation of [i (L)] + with HClO 4 in MC is shown in Figu 4. Th spctum is almost axially symmtic and is vitually idntical to that potd pviously. 39 At th highfild sid of th spctum (g z ), a hypfin stuctu of fiv bands in a ::3:: intnsity atio is psnt, indicativ of coupling to two quivalnt nitogn atoms. Th spctum is simulatd with g valus of g x ).5, g y ).95, and g z ).4, compatibl with th 3d z gound-stat of i III, and a st of hypfin coupling constants of A x ) 5.4 MHz, A y ) 5.8 MHz, and A z ) 5.9 MHz fo th two quivalnt nitogn atoms with I ) (valus dtmind fom simulation of EDOR spcta, vid infa). Contibutions fom oth nucli that do not giv is to solvd hypfin stuctu (55) Gan, M.; Lwinski, K.; Pattk-Janczyk, A.; Rglinski, J.; Siklucka, B.; Spic, M. D.; Szalnic, M. Dalton Tans. 3, (56) Ksting, B.; Sibt, D. Inog. Chm. 998, 3, Inoganic Chmisty, Vol. 4, o. 4, 8

6 Elctonic Stuctu of a Binucla ickl Complx Figu 4. Continuous wav EPR spctum of [i (L)(MC) ] 3+ fomd by oxidation of [i (L)](BF 4) by HClO 4 in MC. Black: xpimntal spctum. Rd: simulation. Expimntal conditions: T ) K, ν ) GHz, P mw ) mw, modulation amplitud.5 mt. Simulation paamts: g x ).5, g y ).95, and g z ).4, linwidths w x ).9 mt, w y ).94 mt, and w z ). mt, nitogn hypfin couplings A x ) 5.4 MHz, A y ) 5.8 MHz, and A z ) 5.9 MHz. a includd in th th lin width paamts w x ).9 mt, w y ).94 mt, and w z ). mt. EDOR Spctoscopy. In Figu 5 (top), th Q-band Davis EDOR spctum fo [i (L)(MC) ] 3+ (-6 MHz) is shown. It ncompasss sonancs fom th potons aound th H Zman fquncy at about 4 MHz. Futhmo, sonancs fom a lag 4 coupling spacd aound A () / spaatd by ν( 4 ) a obsvd and a sonanc at 8.5 MHz that blongs to 4 of th macocyclic ligand. Stongly Coupld itogn Atoms. Ointation-slctd Q-band Davis EDOR spcta btwn and 4 MHz a shown in Figu 5 (bottom). A st of fou signals btwn and 35 MHz is obsvd. Th diffnc in fquncy of bands on and th and two and fou (countd fom lft to ight, s Figu 5) is qual to twic th Zman fquncy of 4 in all of th spcta, indicating that ths signals oiginat fom nitogn that is stongly coupld to th lcton spin ( a ff / > ν ). Th hypfin coupling constant can b ad fom th cnt of th fou bands and vais slightly ov th magntic fild ang ( a ff / 5-9 MHz). Th additional splitting of th EDOR lins is causd by th 4 quadupol intaction (s Expimntal Sction). At high magntic fild (ffctiv g valu.3, cf. Figu 5) th hypfin coupling constant is idntical to that obtaind fom th EPR spctum and, thfo, th stuctu of th fou bands in th EDOR spcta stms fom th sam two stongly coupld nitogn atoms as thos sponsibl fo th hypfin splitting in th EPR spctum. Simulations of th EDOR spcta a includd in Figu 5, and optimizd nucla hypfin and quadupol paamts a givn in Tabl. Th simulations in which only th lcton Zman intaction is considd fo th ointation slction pocdu poduc th positions of th fou xpimntally obsvd bands vy wll and giv asonabl agmnt fo thi intnsitis. Th hypfin intaction is dominantly isotopic (a iso ) MHz) with mino anisotopy (A x )-3. MHz, A y )-.6 MHz, A z )+4.5 MHz) with an angl of 6 btwn th A z and g z pincipal axs. Th lag valu fo a iso as wll as th small angl btwn th A z and g z axs indicats that th 4 nucli a bound axially to i III and hav a lag ovlap with th SOMO, which is dominatd by th 3d z obital. Th nucla quadupol splitting P can obtaind fom th obsvd splitting of ach pai of spctal lins (cf., fo xampl, Figu 5, spctum at g ff ).3). Th optimizd quadupol paamts qq/h and η, dfind as qq/h ) P z, η ) (P x - P y )/P z, amount to -.58 MHz and., spctivly. Fo pu MC th valus fom nucla quadupol sonanc studis 5 a qq/h ) 3.38 MHz and η ).46. Usually, th absolut sign of qq/h cannot b dtmind xpimntally. Howv, in this cas th nitogn hypfin intaction is so lag that it is ith compltly solvd in th EPR spctum o contibuts substantially to th lin width (cf. Figu 4). Und such conditions, th lctonspin tansition inducd by th micowavs bcoms hypfin smislctiv, and th sign of th quadupol splitting can thn b dducd. 58 Th pocdu is dscibd in th Suppoting Infomation, and it tuns out that P z is ngativ. Macocycl itogn Atoms of Intmdiat Coupling. In addition to th stong signals du to axially bound MC, it can b xpctd that th nitogn atoms of th macocycl could also giv is to sonancs, albit with small coupling constants that may not b solvd in th EPR spctum. In od to invstigat th dlocalization of spin dnsity ov th macocyclic -donos, Davis EDOR spcta hav bn codd at X-band (9 GHz) micowav fquncy and th pat of th spctum btwn and MHz is displayd in Figu 6. Th EDOR spcta show a ich stuctu btwn 4 and MHz, and th obsvd bands hav bn simulatd by tntativly assuming that thy a divd fom nitogn (th assignmnt is confimd by DFT calculations and th absolut sign of A and P is infd fom th calculations, vid infa). Simulations a includd in d in Figu 6, and th optimizd simulation paamts a includd in Tabl. Th nucla quadupol paamts qq/h and η did not qui optimization. Th isotopic coupling constant is assssd to b. MHz, small than that fo th axial atoms. Likwis th anisotopic couplings a also small, implying a lag ffctiv distanc fom th unpaid lcton. Th pincipal axs A z and g z a almost ppndicula to on anoth with an angl of 8 (, and th P z and g z pincipal axs a also almost ppndicula (5 ( ). This povids stong vidnc that th sonancs oiginat fom a nitogn atom o nitogn atoms situatd in th quatoial plan of th spin-caying 3d z obital, that is, nitogn atoms of th macocycl. Sinc th hypfin stuctu fo ths nitogn atoms is not solvd in th EPR spctum, th micowav tansitions a not hypfin slctiv in this cas. Th obsvd agmnt btwn simulation and xpimnt with spct to signal intnsitis is btt than that fo th stongly coupld solvnt nitogn atoms. (5) Colligiani, A.; Guib, L.; Haigh, P. J.; Luckn, E. A. C. Mol. Phys. 968, 4, (58) Bihl, R.; Lubitz, W.; Möbius, K.; Plato, M. J. Chm. Phys. 9, 66, 4 8. Inoganic Chmisty, Vol. 4, o. 4, 8 693

7 van Gastl t al. Figu 5. Q-band ESE dtctd Davis EDOR spcta of th axially coodinatd nitogns in [i (L)(MC) ] 3+. Th top spctum was codd at.5 T. Signals btwn 45 and 5 MHz a potons. Th ffctiv g valus a includd in th figu. Black: xpimntal spcta. Rd: simulations. Expimntal conditions: T ) K, ν mw ) GHz, lngth of RF puls 8 µs, lngth of invsion puls ns, dtction squnc ns. Fo simulation paamts, s Tabl. Th nucla Zman (ν), hypfin (A/), and quadupol splitting (P) a indicatd in blu in th spctum at g ff ).3. Tabl. 4 ucla Hypfin and Quadupol Coupling Constants [MHz] and Angls [dg] of th Rspctiv Pincipal Axs with th Pincipal z Axis of th g Tnso fo [i (L)(MC) ] 3+ As Dtmind fom Simulation of th EDOR and ESEEM Spcta and DFT Calculations a hypfin intaction quadupol intaction angl with g z axis angl with g z axis xp DFT xp DFT xp DFT xp DFT 4 (MC) a iso qq/h η..3 A x b P x A y b P y A z P z (i III ) a iso qq/h -.5 c -.5 η.8.8 A x P x A y P y A z P z (i II ) a iso -.4 c -.95 qq/h -.6 c -.6 η..9 A x +.3 c P x A y +.3 c P y A z -.36 c P z a In total, two symmty-quivalnt MC nitogns a psnt, as wll as two quivalnt macocycl nitogns on both th i III and i II cnts. Th calculatd g valus a g x )., g y )., and g z ).5. Sinc th g tnso is naly axial, only th angl with th g z axis is wll dfind. Th os of th paamts usd in th simulation a pincipal valus (. MHz; angls (. b Undfind bcaus of na axiality of th hypfin tnso. c Th sign is infd fom th DFT calculation. ESEEM Expimnts. Macocycl itogn Atoms of Wak Coupling. Sinc th localizd valnc [i L] 3+ macocycl contains two sts of two quivalnt nitogn atoms with on st bing chaactizd by EDOR spctoscopy, lcton spin cho nvlop modulation (ESEEM) xpimnts hav bn pfomd to invstigat th psnc of futh wakly coupld nitogn atoms. Th ESEEM 694 Inoganic Chmisty, Vol. 4, o. 4, 8 tchniqu xhibits a btt snsitivity than EDOR in th low-fquncy ang and is most snsitiv to nucli fo which th hypfin intaction is about qual to th nucla Zman intaction. This phnomnon is calld th xact cancllation condition. 5 Th th-puls ESEEM spcta (fquncy domain) codd at th canonical ointations a shown in Figu. Stong signals a obsvd, and th ESEEM

8 Elctonic Stuctu of a Binucla ickl Complx Figu 8. Stoviw of th singly occupid natual obital, obtaind aft diagonalization of th lcton dnsity matix of [i (L)(MC) ] 3+. Th contou of th 3d z obital on i III can b cognizd. Spin dnsity is also psnt on th nitogn atoms of th axially coodinating solvnt molculs, and a small amount of dnsity is found on th macocycl. Th calculatd pincipal g z axis is paalll to th C axis of th 3d z obital. Th dnsity plot has bn catd with th pogam Molkl. Figu 6. X-band ESE dtctd Davis EDOR spcta of th macocycl nitogns on th i III sid in [i (L)(MC) ] 3+. Black: xpimntal spcta. Rd: simulations. Expimntal conditions: T ) 5K,ν mw ) 9.8 GHz, lngth of RF puls 8 µs, lngth of invsion puls ns, dtction squnc ns. Th ffctiv g valus a includd in th figu. Owing to th lag lin width of th EPR spctum, EDOR spcta could b codd at an ffctiv g valu lag than.5. Figu. X-band 3-puls 4 ESEEM spcta of [i (L)(MC) ] 3+. Expimntal conditions: T ) K, ν mw ) 9.8 GHz, lngth of 9 pulss 6 ns, τ ) 44 ns. A fou stp phas cycl was usd fo acquiing th ESEEM spcta. Th signals stm fom th macocycl nitogn atoms on th i II cnt. spcta display multipl bands blow 4 MHz. Th obsvation of ths bands confims th psnc of nitogn atoms with a wak intaction of about MHz with th unpaid lcton. Attmpts hav bn mad to simulat th ESEEM spcta; howv, th agmnt btwn xpimnt and simulation in this cas is poo. Though th bands in th spcta at th low-magntic-fild sid a asonably wll simulatd, th simulation fails to poduc th stong band at 3. MHz in th spctum at g ).34. Th simulation could not b impovd by futh optimization of nucla hypfin and quadupol paamts. Th ason fo this failu is pobably that th wak coupling of MHz psnts nitogn nucli that a locatd fa away fom th unpaid lcton at positions wh th wav function cofficints of th SOMO a small. Fo such wak couplings, ffcts causd by htognity of th nvionmnt will hav a lag lativ ffct on th magnitud of th coupling constants. Th paamts fo ths nitogn atoms can, thfo, only b usd as an indication of th od-of-magnitud of th couplings. Ths a includd in Tabl and display vy small coupling constants. Th isotopic hypfin coupling constant is ca..4 MHz indicating th psnc of lss than % spin dnsity at ths nitogn atoms. Th quadupol paamts qq/h and η a -.6 MHz and.9, spctivly, and th A z and g z axis a found to b ppndicula, confiming that th sonancs a associatd with th macocyclic nitogns that li appoximatly in th quatoial plan of th 3d z obital. Sinc th coupling constants a small by about an od of magnitud compad to th intmdiatly coupld nitogn atoms, th wakly coupld nitogn atoms a idntifid as thos of th macocycl on th i II sid and th nitogn atoms of th intmdiat coupling as thos on th i III sid. DFT Calculations. On th basis of th obsvd pntt pattn in th EPR spctum, th smallst concivabl modl fo DFT calculations is compisd of two i atoms, th macocycl L and two MC molculs fom th solvnt (Figu ) that a both bound axially to on of th two i atoms. Bcaus of th nonplanaity of th macocycl, it is stically not possibl fo both i atoms to possss a satuatd coodination sph with fou axially bound MC molculs. Aft gomty optimization of this modl in vacuo, th macocycl taind its bowl shap. Th asymmty inducd by th solvnt molculs has a pofound ffct on th calculatd lctonic stuctu. Th Mullikn spin dnsity distibution indicats th psnc of 9% spin dnsity on th six coodinat i cnt bound by two MC ligands (i.., th i III atom) and -3.5% spin dnsity on th oth i (i.., th i II atom), consistnt with th fomal i III /i II chaact aft singl oxidation. Each MC nitogn cais ca..5% spin dnsity, th bidging sulfu atoms -%, ach macocycl nitogn at th i III sid 3.8% and thos at th i II sid -.4% ach. It should b notd that th ngativ spin dnsitis a th sult of spin polaization and that th sum of all atomic spin dnsitis quals %. Th singly Inoganic Chmisty, Vol. 4, o. 4, 8 695

9 occupid natual obital is shown in Figu 8, wh th contou of th 3d z obital at th i III cnt is visibl along with th p z obitals of th MC nitogns. Th g valus, calculatd with th fomalism dscibd pviously, 59 givs g x )., g y )., g z ).5. Compaison with th xpimntal valus of g x ).5, g y ).95, g z ).4 shows that only about 5% of th xpimntally obsvd g shift is poducd by th calculation, vn though th bulk spin dnsity sids on i. In this contxt it should b notd that it is a gnal poblm of th DFT mthod to accuatly poduc g shifts, which a typically undstimatd by about 3%. 59 Th calculatd pincipal z axis of th g tnso is paalll to th C symmty axis of th 3d z obital and ppndicula to th local quatoial plan of th i III cnt. Supisingly, th 9% spin dnsity at i III is patitiond btwn th 3d z obital (%) and th 3d obital (6%). Within th concpts of cystal fild thoy th psnc of th 3d obital in th singly occupid molcula obital is indd compatibl with an longatd octahdon 6,6,6 and is also in-lin with an incasd g z valu, 6 accoding to th xpssions g x ) g - (6E + A )λ/ yz g y ) g - (6E + A )λ/ xz g z ) g - 8A λ/ x -y (8) wh λ is th spin-obit-coupling paamt fo i III (λ < ), th ngy diffncs of th spctiv d obitals with th singly occupid obital a xpssd by, and A and E a th cofficints of th 3d and 3d z obitals in th SOMO, spctivly. ot that ths xpssions hav to b modifid futh whn going fom cystal fild thoy to ligand fild thoy in which th paticipation of th ligand obitals in th SOMO is allowd. Th psnc of ligand obitals may duc th mtal chaact in both th SOMO as wll as in th low-lying doubly occupid obitals that contibut to th g shifts within th ptubation thoy fomalism. Th psnc of 9% spin dnsity is compatibl with th xpimntally dtmind 6 i dipola hypfin coupling constants of MHz (3.6-4 cm - ), -34 MHz (-.3-4 cm - ), and -3 MHz (-.3-4 cm - ). 39 By using singl- Slat functions, th anisotopic hypfin intaction fo a mixd 3d and 3d z gound stat bcoms (s Suppoting Infomation): A ff )- µ 4π γ γ ( a ) (- (A - E ) 3AE 3AE - (A - E ) (A - E ) ) (9) wh a ).59Å, µ is th pmability in vacuum, is th Slat xponnt, and γ and γ a th lcton and nucla gyomagntic atios. Contibutions fom spin dnsity (59) s, F. J. Chm. Phys., 5, (6) Moton, J. R.; Pston, K. F. J. Chm. Phys. 984, 8, van Gastl t al. at th ligands a nglctd. It is notd that th contibutions of th spin dnsitis in th 3d z and 3d obitals hav opposit signs and patially cancl ach oth in q 9. Moov, in gnal, hombicity is intoducd fo a mixd 3d and 3d z gound stat. Howv, an axial tnso may b covd whn A )( 3E. Th pincipal z axis is unaffctd by th admixtu of th 3d z and 3d obitals. Diagonalization with A ).6, E )., and ).65 6,63 givs A x )+4 MHz, A y )-4 MHz, and A z )-4 MHz. This simpl singl- modl pdicts valus that a somwhat too lag, an obsvation that has bn mad bfo in oth nickl systms. 6 This is not supising givn th fact that th obital cofficint is optimizd fo th total ngy of th nickl atom; it is ath unlikly that this valu is also bst fo valuating hypfin intactions of nickl in coodination complxs. Compaison btwn Expimntal and Calculatd itogn Coupling Constants. In th DFT calculation, th sts of quivalnt pais of nitogn atoms a found. Th calculatd nucla hypfin and quadupol constants fo ths nitogn atoms a includd in Tabl. Th stongly coupld axial MC nitogns hav a calculatd isotopic hypfin intaction of a iso )+6.8 MHz, fo th intmdiatly coupld ing nitogn atoms of th macocycl at th i III sid a iso )+6.3 MHz, and fo th wakly coupld nitogn atoms on th i II sid a iso )-.95 MHz. Th xpimntal valus show a simila tnd, thus confiming th assignmnt. A mo dtaild compaison is givn in Tabl. Fo th axial MC nitogns, th anisotopic coupling constants a also in agmnt with xpimnt, as a th angls of th uniqu pincipal axis (A z ) with th g z pincipal axis. All calculatd nucla quadupol paamts of all sts of nitogn atoms, including th angls of ach pincipal axis with th g z axis, ag wll with xpimnt. Th lagst diffnc btwn xpimnt and thoy in this systm concns th hypfin intaction of th wakly coupld macocycl nitogn atoms at th i II sid. Th valu fo a iso diffs by mo than a facto of, and th xpimntally dtmind and calculatd angls with th g z pincipal axis also diff substantially. Th most likly ason fo this discpancy lis mainly in th small spin dnsity psnt at ths nitogn atoms (only -.4%). Th htognity of th local nvionmnt will caus htognity in th lctonic stuctu, which is, to som xtnt, amplifid by th symmtic natu of th macocycl. Futhmo, a small distibution of spin dnsity will thn hav th lagst lativ ffct at thos atoms wh littl spin dnsity is psnt. Taking into account that th ESEEM ffct dpnds citically and nonlinaly on how wll th xact canclation condition is fulfilld, th poo agmnt btwn th ESEEM spcta and simulation on on sid, and also th somwhat mdioc agmnt btwn xpimnt and thoy fo th hypfin couplings is likly ootd in this htognity. Th nucla quadupol intaction, which is dtmind by th lctic fild gadint causd by all lctons at nitogn, is not snsitiv to ths htognitis. (6) Abagam, A.; Blany, B. Elcton Paamagntic Rsonanc of Tansition Mtal Ions; Oxfod Univsity Pss: Oxfod, Inoganic Chmisty, Vol. 4, o. 4, 8

10 Elctonic Stuctu of a Binucla ickl Complx Th calculatd and xpimntal quadupol paamts fo th wakly coupld nitogn atoms a found to b in good agmnt. Conclusions and Implications fo [if] Hydognass. Th complx [i (L)(MC) ] 3+ has bn studid by EPR, EDOR, and ESEEM spctoscopis and DFT calculations. Although th cystal stuctu of th ttanucla complx [(i (L)ClO 4 ) (µ -Cl) ] x+ (x ) -) and th solution stuctu of th binucla complx [i (L)(MC) ] 3+, infd fom spctoscopic studis, a supficially vy diffnt, th bowl shapd stuctu of th macocycl obsvd in th X-ay analysis has also bn obtaind aft DFT gomty optimizations fo [i (L)(MC) ] 3+ and [i (L)(MC) ] + in vacuo. It is thfo likly that th solution stuctu is also bowl shapd. In both th cystal stuctu as wll as in th DFT calculations, oxidizd i cnts a psnt, though in th fom cas an assignmnt of fomal oxidation stats to th i cnts was not possibl. Th lcton-spin dnsity distibution has bn invstigatd by xamining th hypfin coupling constants of nitogn nucli magntically coupld to th unpaid lcton, and six nitogn atoms hav bn spctoscopically idntifid. Ths a dividd into th sts of two quivalnt nitogn atoms. Th st of stongly coupld nitogn atoms (a iso ) 53.4 MHz) a assignd to th atoms of th MC molculs that a axially bound to i III. Th st of nitogn atoms with intmdiat coupling (a iso ). MHz) a assignd to macocyclic nitogn atoms dictly bound to i III in th quatoial plan of th 3d z obital. Th thid st (a iso )-.4 MHz) of coupling constants is assignd to th macocycl nitogn atoms at th i II sid. By compaison with DFT calculations, it can b concludd that th systm isai III -i II systm with localizd valncs in which 9% lcton spin dnsity is found at i III and -3.5% at i II. Th 9% spin dnsity at th i III cnt is patitiond btwn th 3d z obital (%) and th 3d obital (6%). Th maining spin dnsity is dlocalizd ov th macocycl, -% at ach sulfu atom and 3.8% at th macocycl nitogn atoms of i III. Owing to th symmtic natu of th macocycl, th amount of dlocalization ov th macocycl changs with htognity of th local nvionmnt. This ffct is most appant fo th wakly coupld nitogn atoms at th i II sid. With spct to [if] hydognas, th psnc of.5% lcton-spin dnsity at ach axial ligand is an impotant finding. It is blivd that th sit of catalytic activity in [if] hydognass is th dox-activ i cnt. 6 Fom th spctoscopic masumnts and analysis psntd h, it bcoms cla that th intaction btwn i III and its axial ligand distibuts som of th lcton dnsity away fom th ligand and towad th mtal cnt. 64 In this way lcton-spin dnsity aiss at th ligand and th chmical bonds of th axial ligand bcom polaizd, as is obsvd in th DFT calculations. This is an impotant finding and may hint at a mchanism fo [if] hydognass wh th substat H binds to i and th H bond bcoms polaizd. In od to tst th validity of this hypothsis, additional calculations hav bn pfomd on a [i (L)(MC)(H )] 3+ molcul wh on axial MC is atificially placd by H. Indd, th H bond bcoms polaizd, vn mo so than in th cas of MC, and up to % spin dnsity is found on th distal hydogn atom. Simila ffcts a obsvd fo mo hydognas-lik systms with [i 3+ (SH - ) 4 (OH - )(H )] coodination, wh th distal H atom acquis 6% spin dnsity, and in a faithful modl of th activ sit in [if] hydognas (s Suppoting Infomation), wh 6% spin dnsity is found at th distal H atom. Though this xamination dos not pov that molcula hydogn actually binds at th axial position to nickl, if it dos, th H bond bcoms stongly polaizd, which in tun facilitats th htolytic splitting of H. Acknowldgmnt. W thank th EPSRC UK and th Univsity of ottingham fo suppot. M.S. gatfully acknowldgs cipt of a Royal Socity Wolfson Mit Awad and of a Royal Socity Lvhulm Tust Snio Rsach Fllowship. This wok was suppotd by th Max Planck Gsllschaft and by th EU (SolaH). Suppoting Infomation Availabl: Cystallogaphic Infomation Fil (CIF) fo [(i (L)ClO 4 ) (µ -Cl) ] x+, coodinats of th modl fo [i (L)(MC) ] 3+ usd in th DFT calculations, dtmination of th absolut sign of th quadupol intaction fo th stongly coupld nitogn atoms and th hypfin tnso fo a mixd 3d z,3d SOMO, and spin dnsity plots in DFT calculations with th aim to invstigat th binding of H. This matialisavailablfofchagviathintntathttp://pubs.acs.og. IC8383X (6) Clmnti, E.; Raimondi, D. L. J. Chm. Phys. 963, 38, (63) Rosicky, F.; Mak, F. Tho. Chim. Acta 99, 54, (64) Stnson, P. A.; Boad, A.; Main-Bca, A.; Blak, A. J.; Davis, E. S.; Wilson, C.; McMast, J.; Schöd, M. Chm. Eu. J. 8, 4, Inoganic Chmisty, Vol. 4, o. 4, 8 69

11 Suppoting infomation I. Cystallogaphic Infomation Fil (CIF) fo [(i (L)ClO 4 ) (µ -Cl) ] x+ x+ Tabl S. Cystal Data and Stuctu Rfinmnt fo [((µ -ClO 4 )(i (L)) (µ -Cl)] Idntification cod ni3ns Empiical fomula C 48 H 8 Cl 6 8 i 4 O S 4 Fomula wight 83.5 Tmpatu 5() K Wavlngth.3 Å Cystal systm Ticlinic Spac goup P- Unit cll dimnsions a = 3.39() Å α = 9.43() b = 3.6() Å β =.58() c = 4.5() Å γ = 9.66() Volum 49.5(6) Å Z Dnsity (calculatd).9 Mg/m 3 Absoption cofficint.385 mm F() 46 Cystal siz.8 x.6 x. mm 3 Thta ang fo data collction.8 to.5 Indx angs 6<=h<=6, <=k<=, 3<=l<=9 Rflctions collctd 5639 Indpndnt flctions 99 [R int =.34] Compltnss to thta = % Absoption coction Smi-mpiical fom quivalnts Max. and min. tansmission. and. Rfinmnt mthod Full-matix last-squas on F Data / staints / paamts 99 / 8 / 5 Goodnss-of-fit on F. Final R indicis [I>σ(I)] R =.588, wr =.36 R indics (all data) R =.6, wr =.64 Lagst diff. pak and hol.38 and..å 3

12 II. Coodinats [Å] of th modl fo i (L)(MC) usd in th DFT calculations. i... S C C C C C C C C C C C C H H H H H H H H H H H H H S i C C C C H C C C C H H C C H H H H H C H H C H H H C

13 C H H H C C H H H

14 III. Th absolut sign of th nucla quadupol splitting Th absolut sign of th quadupol intaction can b dtmind fo th stongly coupld MC nitogn atoms. Consid th intnsity distibution of th signals in th EDOR spcta codd at th low-fild (g ff =.) dg of th EPR spctum, cf. figu 5. Without hypfin smi-slction, fou bands with about qual intnsity a xpctd, as is shown in th simulation of figu 5. Howv, whn th ngy lvl diagam fo an S = ½ systm coupld to a nitogn (I( 4 ) = ) with dominant hypfin intaction is considd (s figu S), only th ( M S = ±, M I = +) EPR tansition is sonant at th low-fild dg of th EPR spctum. Thfo, in th EDOR spctum, th tansitions M I = +, indicatd in d in figu S will dominat and th tansitions M I = will b attnuatd. This is th cas fo th two innmost signals in th EDOR spctum, and such a pattn can only b poducd by taking a positiv valu fo P x in Figu S. If P x is ngativ, th outmost two signals will dominat th EDOR spctum. By th sam agumnts, th tansitions M I = will dominat th EDOR spctum fo th magntic fild st na g z (s Figu S). Sinc th EDOR spctum at g ff =.3 displays fou signals fo which th innmost two dominat th spctum, P z has to b ngativ. 4

15 Figu S. Engy lvl diagam, including lcton and nucla Zman, hypfin and quadupol intactions (not to scal) of th lcton and nitogn nucla spin lvls at lowfild (B g x ) and high-fild (B g z ) dg of th EPR spctum. Th vtical aow indicats th micowav tansition. At both dgs, hypfin-(smi)slction occus and th two innmost signals in th EDOR spctum bcom dominant only fo P x > MHz and P z < MHz. 5

16 IV. Hypfin tnso fo a mixd 3d z, 3d SOMO IV.. Dfinitions Th SOMO as found in [i L(MC) ] 3+ is givn by Ψ = A + E z (.) wh A and E a th wavfunction cofficints. A and E a spin dnsitis in th and z obitals. Gnal Slat obital (n = 3) xpssions fo th 3 and d obitals d 3 z z a na na = = (.) ( 3z ) π ( 3) 8 a na 3z 9 π 3 na = = (.3) z wh is th Slat xponnt and and a th nomalization factos. z Th anisotopic hypfin intaction (s,.g. Caington, A. and McLachlan, A.D., Intoduction to magntic sonanc, Hap and Row, w Yok, 96) is givn by v A 3x ( ) = γ γ 3 3y 3yz 5 3xz 3 3yz 3xz 3z (.4) wh γ and γ a th lctonic and nucla gyomagntic atios. ot that th tac of (.4) is zo. Th hypfin tnso of th spin Hamiltonian is givn by A = Ψ A Ψ (.5) 6

17 IV.. Hypfin tnso fo a pu 3d z SOMO (A = ) Fo a pu 3d z SOMO, quation (.5) bcoms A = E z A (.) z Th off-diagonal lmnts a unvn functions of x, y and/o z. Sinc th 3d z obital (.3) is an vn function of x, y and z, all off diagonal lmnts valuat to. Th A zz tm valuats to A zz ( 3 ) na z 5 3z = γ γ E (.) z Th intgal can b valuatd aft tansfomation into sphical coodinats ( 3cos θ ) ( 3cos θ ) 3 na Azz = γ γ E d sinθdθ dϕ (.3) z All th intgals a tivial. Th ϕ intgation givs π. Th θ intgal can b wittn as ( 3x ) 3 96 dx = (.4) 5 Th intgal valuats to d 3 na 3 3 = na d 3 na na = na = 6 4 P (.5) wh th intgal is abbviatd as P to sav spac in th coming fomulas. Substitution of th intgals into (.8) givs A zz ( 3) a 96 3 a = γ γ E P 9 π = γ γ E P (.6) π 5 8 ( 3) 5 Th A xx and A yy lmnts can in pincipl b calculatd similaly. Howv, it is known that th tac of th anisotopic tnso (.4) is (A xx + A yy + A zz = ) and it is tivial to s that th A xx and A yy tms giv xactly th sam intgal. Thus, A xx = A yy = ½A zz. Th total hypfin tnso fo a pu d SOMO bcoms 3 z A = γ γ E a P (.) Th tnso is axial and colina with th z axs of th st of d obitals.

18 IV.3. Hypfin tnso fo a pu 3 d SOMO (E = ) ow w div th hypfin tnso in th cas of a pu valuats to 3 d obital. Equation (.5) A = A A (3.) Th off diagonal lmnts a again zo, bcaus th intgand is an unvn function of x, y and/o z. Th A zz lmnt bcoms A zz na 3z = γ γ A x y (3.) 5 Tansfomation into sphical coodinats lads to ( 3cos θ ) 3 na 4 A = γ γ sinθ θ d zz A d d ϕ sin θ sin ϕ cos ϕ (3.3) Th intgal is qual to (.5) and th ϕ intgal valuats as π π d ϕ sin ϕ cos ϕ = (3.4) 4 Th θ intgal simplifis to ( x ) ( 3x ) 64 dx = (3.5) 5 Substitution of th intgals into (3.3) followd by nomalization givs A zz ( 3) a 6π 3 a = γ γ A 4 P 8 = γ γ A P (3.6) 8 π 5 ( 3) 5 ot that xcpt fo th sign, th xpssion (3.6) quals (.6). Th A xx and A yy lmnts a again ith valuatd similaly, o by making us of th taclssnss of (.4). Thus, A xx = A yy = ½A zz. Th total hypfin tnso fo a pu 3 d SOMO is A = γ γ A a P (3.) Th xpssions (.) and (3.) only diff in sign, and th cofficints A and E. Th tnso is again axial and colina with th z axs of th st of d obitals. 8

19 IV.4. Hypfin tnso fo mixd 3d z, 3d SOMO Following th sam pocdu again and using (.) as th SOMO, quation (.5) bcoms A z + A A( ) + AE A( ) z = E z A (4.) Th fist two tms hav bn valuatd in sctions and 3. Th coss tm AE A( ) z now has to b valuatd. Th coss tm only contibuts to th A lmnt of (.4), sinc all oth lmnts giv an intgand that is unvn with spct to ith x, y o z. Th A lmnt bcoms A zz 3 ( 3 ) na z 5 = γ γ AE (4.) z Tansfomation into sphical coodinats givs A zz = γ γ AE = γ γ 6AE (4.3) z z P d 3 na 4 sinθdθ dϕ sin θ sin ϕ sinθdθ dϕ 4 ( 3) sin θ sin ϕ cos ϕ( 3cos θ ) cos ϕ( 3cos θ ) Th θ and ϕ intgals qual (3.4) and minus (3.5). Thus, A zz = γ γ 6AE = γ γ AEP 3 z π P 4 a 3 8 ( 3) = γ γ 6AEP 3 a π ( 3) 8 π (4.4) Th total hypfin tnso bcoms A = γ γ P a ( 3) ( A E ) 3AE 3AE ( A E ) A E (4.5) Thus, th coss tm of (4.) causs a ointation of th pincipal x and y axs of A. Th pincipal z axis mains paalll to th z axis of th d obital. Diagonalization of (4.5) givs ignvalus (dnotd by pims) 3 z 9

20 A' A' x y = γ γ P = γ γ P A' z ( 3) 8 ( 3) 8 = γ γ P a a a ( 3) 8 ( A E ) ( A E ) ( A E ) 3AE 3AE (4.6) In gnal, th additional tms caus hombicity, i.., Howv, fo th cas A' x and A = ± 3E and E = ± 3A, th tnso bcoms axial. A' y a no long qual.

21 V. Stuctus (lft) and spin dnsity plots (ight) in DFT calculations with th aim to invstigat th binding of H. V.. [i (L)(MC)(H )] 3+ V. [i 3+ (SH ) 4 (OH )(H )] V.3 Hydognas activ sit, [F + (C ) (CO)i 3+ (SCH CH 3 ) 4 (ΟΗ )(C 4 H 3 CH 3 )(Η )]