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The First Hydrides of Boron

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The First Hydrides of Boron UNIVERSIDADE DE LISBOA FACULDADE DE CIÊNCIAS SECÇÃO AUTÓNOMA DE HISTÓRIA E FILOSOFIA DAS CIÊNCIAS UNDER THE CARBON SPELL: DIBORANE’S PUZZLING STRUCTURE AND THE EMERGENGE OF BORON CHEMISTRY Nuno Manuel Castanheira de Figueiredo Dissertação orientada pela Professora Doutora Ana Simões, da Secção Autónoma de Filosofia e História das Ciências da Faculdade de Ciências da Universidade de Lisboa. MESTRADO EM HISTÓRIA E FILOSOFIA DAS CIÊNCIAS 2011 Resumo Este trabalho apresenta o primeiro estudo sistemático da história da química dos boranos, compostos de boro e hidrogénio cujas estruturas e natureza das ligações químicas desafiaram de forma irredutível a teoria da ligação química até aos anos cinquenta do século XX. Actualmente, a química do boro é um dos mais promissores ramos da química, com um vasto leque de aplicações às indústrias química e farmacéutica, à nano-tecnologia e à medicina. Neste último ramo, destacam-se as aplicações na luta contra o cancro e no desenvolvimento de medicamentos com um elevado grau de especificidade e inovação. Num futuro próximo, espera-se que a química do boro seja capaz de operar uma verdadeira revolução social, posicionando-se como uma poderosa alternativa à química do carbono que será capaz de oferecer todo um novo mundo de aplicações inéditas. Estas são o resultado da fascinante capacidade do átomo de boro para se ligar de formas surpreendentes e formar complexas estruturas que se baseiam em compostos de boro e hidrogénio (boranos). A grande apetência do boro para se ligar ao oxigénio impede que os boranos existam na natureza. A grande susceptibilidade destes à acção da humidade e do ar torna-os especialmente instáveis e difíceis de manusear e preservar. Embora apresente pela primeira vez uma descrição detalhada das principais contribuições feitas pelos pioneiros desta química ao longo do século XIX, este trabalho foca-se na era moderna da história dos boranos, que pode considerar-se ter começado com o trabalho de Alfred Stock na Alemanha. A obra de Alfred Stock é aqui amplamente descrita e discutida. São descritos o carácter inovador e os detalhes técnicos das suas investigações, dando-se especial relevo àquelas que se viriam a revelar importantes na busca pela estrutura dos boranos. É também salientado o papel instrumental da analogia entre o carbono e o borano na obra de Stock. Efectivamente, Stock dedicou-se ao estudo dos boranos porque a analogia entre carbono e boro o levou a acreditar que poderia vir a descobrir uma química do boro tão fértil quanto a do carbono. Recorrendo às suas inovações técnicas, Stock conseguiu isolar pela primeira vez vários boranos. Com efeito, entre 1912 e 1914, o tetraborano B 4H10 , o diborano B 2H6 e decaborano B 10 H14 foram identificados e estudados. As suas fórmulas 1 puseram imediatamente em causa a ideia de que o átomo de boro era trivalente nas suas ligações e que tinha sido estabelecida ao longo do século XIX pelo estudo de outros compostos de boro. Pelo contrário, estavam mais de acordo com um boro tetravalente, o que se adequava à crença de Stock numa semelhança fundamental entre as químicas do boro e do carbono. Em 1916, Gilbert Newton Lewis publicou a sua teoria, segundo a qual a ligação química entre dois átomos era estabelecida através de diferentes graus de partilha de pares de electrões. No entanto, os boranos não possuíam electrões suficientes para sustentar as suas ligações através da partilha de pares de electrões, quaisquer que fossem as suas estruturas. O fenómeno ficou conhecido como “insuficiência electrónica” (electron deficiency). Seguiram-se inúmeras tentativas para tentar lidar com este fenómeno, em especial no borano mais simples, o diborano B 2H6. No entanto, este mostrou-se irredutível e todas as estruturas propostas implicavam, ou estruturas singulares no panorama químico, ou o abandono de princípios fundamentais da emergente teoria da ligação química, como a ligação por emparelhamento de electrões ou o octeto de electrões. Neste trabalho demonstra-se inequivocamente a grande relevância teórica atingida pelos boranos na época, com tentativas de resolução do problema por parte de todos os principais contribuidores para a teoria da ligação química. No início dos anos vinte, Stock isolou os pentaboranos B 5H9 e B 5H11 e o hexaborano B6H10 , o que o obrigou a rever a tetravalência do boro nos boranos. No entanto, em 1925, Herman Mark e Erich Pohland (um dos colaboradores de Stock) tinham analisado uma amostra cristalizada de diborano (um gás à temperatura ambiente) por difracção de raios X e concluido que este apresentava uma grande semelhança com o etano C 2H6. Este resultado levou Stock a manter a tetravalência do boro no diborano, mas foi obrigado a aceitar a coexistência da tetravalência e da trivalência do boro nos boranos. Com o avançar do trabalho de Stock e do seu grupo, em especial com as suas investigações sobre os compostos de sódio e de amoníaco do diborano, pareceu ficar evidente que dois dos seis átomos de hidrogénio do diborano tinham um papel especial na sua estrutura. Esta interpretação levou a que apenas duas das muitas propostas para a estrutura do diborano fossem consideradas como concordantes com os factos empíricos: a estrutura semelhante ao etano (BH 3) – (BH 3) proposta por Nevil Vincent Sidgwick em 1927 e apoiada e modificada por Linus Pauling em 1931, em que cada átomo de boro estaria ligado a um átomo de hidrogénio por um único electrão (one-electron bond), e a 2 - - + estrutura semelhante ao eteno [H2B = B H2]2H , em que os dois protões se encontravam inseridos dentro da nuvem electrónica dos dois átomos de boro, e que foi proposta em 1928 por Egon Wiberg, um dos alunos e colaboradores de Stock. Em 1931, Stock, Wiberg, Hans Martini e August Nicklas publicaram um estudo sobre a electrólise do composto de amoníaco do diborano numa solução de amoníaco. Esta reacção foi explicada através de um mecanismo que apoiava fortemente a estrutura de Wiberg e contribuíu decisivamente para que esta ganhasse uma preponderância momentânea no debate sobre a estrutura do diborano. Embora reconhecendo que a estrutura de Wiberg se adequava de forma notável aos resultados experimentais, incluindo resultados não publicados que haviam sido obtidos por Hausser sobre o espectro de absorção ultra-violeta do diborano, Stock preferiu manter-se de fora do debate, acreditando que a sua resolução não podia ser atingida através dos conceitos de uma teoria da ligação química que havia sido desenvolvida a partir da química enganadoramente simples do carbono. Entretanto, a consciência de que os rendimentos extremamente baixos dos métodos de produção de Stock impediam uma investigação mais rápida e intensiva que pudesse levar à resolução do problema, levou a uma busca de novos métodos por volta de 1930. Quer Stock, quer Bertram Steele na Austrália conseguiram pequenos avanços no método de Stock, baseado na acção de ácido clorídrico (HCl) sobre um composto de magnésio e boro. No entanto, os novos métodos de ambos continuavam a usar soluções aquosas de ácidos, o que, dada a sensibilidade dos boranos à hidrólise, impedia um aumento significativo dos rendimentos das reacções de produção dos boranos. A solução foi encontrada inadvertidamente por Hermann Irving Schlesinger e Anton Behme Burg em 1931, do departamento de química da Universidade de Chicago. A descoberta foi feita no contexto do trabalho doutoral de Burg, sob a orientação de Schlesinger, e a propósito de uma tentativa falhada de produzir boro puro em que foi detectada a presença de grandes quantidades de diborano. O novo método não envolvia a utilização de solucões ácidas mas sim a aplicação de uma corrente eléctrica e permitia obter rendimentos incomparavelmente maiores aos de Stock (55% contra os 3 a 5% de Stock). Schlesinger e Burg iniciaram então investigação que visava a produção dos restantes boranos a partir do diborano, agora mais acessível. Com muito poucas excepções inteiramente ocasionais, como a de Bertram Steele, os grupos de Stock e de Schlesinger foram até aos anos quarenta os únicos a sintetizar os boranos, tornando-se 3 os centros difusores das amostras necessárias aos vários estudos que se fizeram sobre o diborano. O problema estrutural constituíu a motivação de Schlesinger e Burg para se dedicarem a este tipo de química e o seu programa evidencia-se precisamente pela completa dedicação à sua resolução. Schlesinger e Burg acreditavam que o diborano tinha uma estrutura semelhante à do etano e que, portanto, consistia na ligação de dois radicais BH 3 entre si. Mas Stock havia falhado na decomposição do diborano em duas moléculas BH 3 e Schlesinger e Burg orientaram uma parte do seu programa de pesquisa para o estudo da estabilidade da ligação entre os dois átomos de boro. Também estudaram reacções que evidenciaram a incorporação de moléculas BH 3 que provinham do diborano, pretendendo assim estabelecer a molécula BH 3 como a unidade estrutural que permitia compreender as reacções do diborano com outros compostos e, indirectamente, estabelecer a realidade estrutural de dois grupos BH 3 no diborano. As investigações de Schlesinger e Burg prosseguiam um pouco a contra-corrente, devido à vantagem de que usufruía o modelo de Wiberg por conseguir explicar a electrólise do composto de amoníaco do diborano. A situação agravou-se significativamente com a publicação por Wiberg de um extenso artigo de revisão em 1936. Wiberg admitia aí que a química por si só não podia resolver a questão e invocava como argumentos definitivos em favor da sua teoria uma série de medições de propriedades físicas do diborano, como o comportamento magnético ou o momento dipolar, que entretanto haviam sido realizadas por vários autores interessados em contribuir para esclarecer o problema estrutural do diborano. No entanto, em 1936, Simon H. Bauer, do Instituto de Tecnologia da Califórnia, iniciou uma série notável de estudos de vários boranos através da recente técnica de difracção de electrões, que havia sido desenvolvida por Pauling a partir da técnica criada por Mark e Wierl em Berlim e que viria a ser usada num extenso programa de análise estrutural de centenas de moléculas.
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