Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 51. Alkylidyne(carbaborane)tungsten-gold and -rhodium complexes; crystal structures of [AuW(µ-CR)(CO)2(PPh3)(η-5C2B9H9Me2)], [RhW(µ-CR)(CO)2(PPh3)2(η5-C2B9H9Me2)], and (RhW[µ-CR)(CO)2(PPh3)2{η5-C2B9(C7H9)H8Me2}](R = C6H4Me-4)

Abstract

Reactions between the bromo(alkylidyne)tungsten complex [W(CR) Br(CO)₄](R = C₆H₄Me-4) and the salts Na₂[7,8-C₂B₉H₉R′₂](R′= Me or H), followed by treatment with [N(PPh₃)₂]Cl afford the compounds [N(PPh₃)₂][W(CR)(CO)₂(η5-C₂B₉H₉R′₂)](1a, R′= Me; 1b, R′= H). The salt [NEt₄][W(CMe)(CO)₂(η⁵-C₂B₉H₉Me₂)](1c) was prepared from [W(CMe)Br(CO)₄] and Tl₂[7,8-C₂B₉H₉Me₂], with subsequent addition of [NEt₄]Cl. Treatment of (1a) or (1b) with [AuCl(PPh₃)], in the presence of TIPF₆ yields the gold–tungsten complexes [AuW(µ-CR)(CO)₂( PPh₃)(η⁵-C₂B₉H₉R′₂)](2a, R′= Me; 2b, R′= H). Similarly, (1c) with [AuCl( PPh₃)] affords [AuW(µ-CMe)(CO)₂( PPh₃)(η⁵-C₂B₉H₉Me₂)](2c). The structure of (2a) has been established by X-ray diffraction. The Au–W bond [2.780(8)Å] is asymmetrically bridged by the CC₆H₄Me-4 group [µ-C–Au 2.19(3), µ-C–W 1.88(3)Å]. Treatment of [AuCl(tht)](tht = tetrahydrothiophene) with (1a) gives the salt [N( PPh₃)₂][AuW₂(µ-CR)₂(CO)₄(η⁵-C₂B₉H₉Me₂)₂](3). The rhodium–tungsten compound [RhW(µ-CR)(CO)₂(PPh₃)₂(η⁵-C₂B₉H₉Me₂)](4a) has been prepared by two routes: from the reaction between (1a) and [RhCl(PPh₃)₃] or from (1a) and [Rh(PPh₃)₂(cod)][PF₆](cod = cyclo-octa-1,5-diene). The latter with (1b) yields [RhW(µ-CR)(CO)₂(PPh₃)₂(η⁵-C₂B₉H₁₁)](4b). Compounds (1a) and (1b) react with [Rh(PPh₃)₂(nbd)][PF₆][nbd = norbornadiene (bicyclo[2.2.1] hepta-2,5-diene)] to give the complexes [RhW(µ-CR(CO)₂(PPh₃)₂{η⁵-C₂B₉(C₇H₉)H₈R′₂}][5a,R′= Me; 5b, (two isomers), R′= H]. The structures of (4a) and (5a) have been established by X-ray diffraction. Both molecules have Y-shaped WRhP₂ cores with the Rh–W bonds [4a, 2.681 (l), 5a, 2.662(2)Å] bridged by CC₆H₄Me-4 groups [4a, µ-C–Rh 2.068(5), µ-C–W 1.880(6); 5a, µ-C-Rh 2.12(2), µ-C–W 1.84(2)Å], and semibridged by a CO ligand [W–C-O, 4a, 160.2(5); 5a, 156( 1 )°] In (4a), the tungsten atom is ligated by a terminal CO group and the η⁵-C₂B₉H₉Me₂ cage. In (5a), the tungsten also carries a terminal CO group but the carbaborane ligand has a C₇H₉[nortricyclene (tricyclo[2.2.1.0^2,6] heptane)] fragment attached to the unique boron atom in the open face of the cage. Thus in the formation of (5a) an unprecedented hydroboration reaction has occurred, a cage B–H bond having added to the norbornadiene ligand with concomitant C–C bond formation. In contrast, treatment of the norbornadiene-containing salt (Rh(dppe)(nbd)[PF₆](dppe = Ph₂PCH₂CH₂PPh₂) with (1a) gives [RhW(µ-CR)(CO)₂(dppe)(η⁵-C₂B₉H₉Me₂)](4c). Reactions of (4a) with PEt₃, PMe₂Ph, dppe, and CO are reported. Examination of the ¹³C-{¹H} and ³¹P-{¹H} n.m.r. spectra of the complexes (4) and (5) reveal that they undergo dynamic behaviour in solution and possible mechanisms are described and discussed.