I have come to believe that thought leadership and content marketing are not the same thing. Content marketing is the "marketing and business process for creating and distributing relevant and valuable content to attract, acquire, and engage a clearly defined and understood target audience – with the objective of driving profitable customer action" (Content Marketing Institute).
On the other hand, a thought leader can be defined as "an individual or firm that is recognized as an authority in a specialized field and whose expertise is sought and often rewarded"; accordingly, thought leadership is the work product of those authorities. Thought leadership consists of ideas that require attention, that offer guidance or clarity and that can lead people in unexpected, sometimes contrarian directions (think of Seth Godin). Done well, thought leadership build trust in your brand, because buyers trust experts -- and that trust is an incredibly brand important for risk-adverse buyers.
In the end, both thought leadership and content marketing can build your awareness and brand, but true thought leadership is much rarer. Also, if you want your firm to create thought leadership, then you must in fact hire thought leaders.
Read more on at Bloomgroup's post Content Marketing vs. Thought Leadership Marketing.